Acquire a piano.
Most teachers recommend that you 
purchase an actual piano to keep and practice on at home. Pianos vary 
widely in style, size, and cost; for most people, a medium-sized upright
 piano is ideal, as it takes up less space than a grand piano and has 
much better sound than the most compact styles. Bear in mind that 
although it's possible to find pianos for cheap and sometimes even for 
free, it is dangerous and difficult to move a piano from place to place.
 Be sure you have a professional delivery service lined up to help you 
move your instrument.
When you acquire a used piano for little or no money off the private
 market, it's likely that you'll need to get it tuned and possibly even 
repaired before it will be usable. Check your local business listings to
 find someone who's willing to make a house call and assess your piano's
 condition.
It's acceptable to purchase a high-quality electronic keyboard if 
you must (for example, if you live in a tiny studio apartment), but it 
isn't recommended.
 You'll have more trouble learning proper posture and 
hand positioning than you would with an actual piano, and you'll never 
be able to exert as much control over the sound of a keyboard as you can
 with a decent piano.
Acquire accessories.
Once your piano is set in place 
and has been tuned and examined by a professional, it's time to get a 
bench and some music to play. Many pianos come with a bench; otherwise, 
they can be found at thrift stores and music stores. Try to get an 
adjustable bench, as bench height is very important to ensuring proper 
posture. Don't use a dining room chair or an ottoman as a substitute 
unless it naturally places you at the ideal height for playing.
Ask your local music store for recommendations on basic, 
easy-to-play music books. It's likely the clerk will know of at least a 
couple of books you can use. Try to get one book that encapsulates 
beginner advice in the front and contains scales and arpeggios, and one 
book with simple, complete songs to practice, such as old folk songs.
If you have trouble keeping a rhythm, buy a metronome. A metronome 
sits atop your piano and ticks like a loud clock at a pace you set for 
it. It's a useful aid for maintaining a steady tempo as you begin to get
 better at playing.
Set up for proper posture.
Sit on your piano bench 
with your music books on the sill of the piano, above the keyboard. 
Point your forearms towards the keys until they're parallel to the 
floor. If your bench is at the proper height, your fingers should gently
 arc downward and rest on the tops of the keys, without you having to 
bend your wrists or raise or lower your arms. Adjust the bench height 
until you feel no strain in your arms or shoulders when you sit at the 
piano.
Rest your feet flat on the ground, a comfortable distance apart, 
with the toes pointing straight ahead. Sit with a straight back - your 
shoulders shouldn't hunch forward, and your spine should be straight. 
Scoot in until your hands rest on the keys without having to stretch 
your arms. You should be able to move your feet forward to the pedals 
and back again without putting any strain on your upper legs.
If you don't have an adjustable piano bench, or it won't adjust high
 enough to make you comfortable, it's acceptable to use pads or pillows 
to further raise the height of your seat. Just be sure they're even in 
terms of thickness, and stable enough that you won't have to worry about
 them slipping as you play.
Double check your hand position. You should be seated
 at the center of the keyboard. Each of your ten fingers should rest on 
one white key. Your right thumb rests on the white key just to the left 
of a group of two black keys in the middle of the keyboard, which is the
 C note. Each finger on your right hand rests on the next following 
white key, thus D, E, F, and G. Your left hand follows the same pattern 
one octave down, only reversed: the left pinky should rest on C, and the
 left thumb should rest on G. There should be two white keys (A and B) 
between your thumbs.
The C note that anchors your right thumb is in the center of the 
keyboard, and as such is often called “middle C.” It's common for 
beginning players to use a sticker or a piece of tape to mark middle C. 
Just be sure it's something you can eventually clean off as you get 
better.
Learning the notes at the center first is conventional because a 
pianist must normally sit at the center of the keyboard to be able to 
reach every high and low note without standing up and moving.
Learn about the keyboard.
The keyboard of a piano 
repeats its notes from top to bottom across several octaves. This means 
that the notes change from low (left side) to high (right side), but 
don't vary in pitch. There are twelve notes a piano can produce: seven 
white key notes (C, D, E, F, G, A, B) and five black key notes (C-sharp,
 D-sharp, F-sharp, A-flat, and B-flat). Playing the white key notes from
 C through B and back to C creates a regular eight tone octave scale in C
 major; playing the black key notes from C-sharp (next to the white C 
key) through B-flat creates a pentatonic (five tone) scale. You can play
 every key (both black and white) from C to C in order to create a 
chromatic or twelve-tone scale.
The piano is tuned for C major, since it's a very common key. 
However, other keys can be played by mixing black and white keys to get 
the proper sharps and flats for each note. Any of the keys normally seen
 in sheet music can be played on a piano, making it a very versatile 
instrument.
It's easy to double-check a piano's tuning by playing pairs of the 
same note at different octaves. The notes should sound identical; if 
they don't, one or both of them is sharp or flat and needs to be 
re-tuned.
Play some notes.
Starting with middle C, gently but 
firmly push down on a key to produce a note. Take some time and try 
pushing slower and faster, softer and harder, until you have an idea of 
the kind of control you can exert over the sound of the piano. Without 
moving your hands, play all ten notes under your fingers. Note that some
 (such as the pinky) are more difficult to play loudly, while others 
(like the thumb) take practice to play softly.
Play black keys as well. Generally speaking, black keys are played 
by moving the appropriate finger up and to the right, except in the case
 of the thumb, since it's difficult to move the thumb up to play a black
 key without ruining your hand position. From the middle C position, 
your pointer finger can move left and right to play C-sharp and D-sharp,
 respectively.
Play scales.
Try playing notes starting with the 
pinky C of your left hand, all the way up to the thumb C of your right 
hand. Play each white key in order. Once you reach your left thumb, 
stretch it to play the next note up (A), and then stretch your right 
thumb to play the next after that (B), returning your thumb to middle C 
to finish the scale. Practice this until it begins to get easier – 
stretching your fingers is just as important as moving your hands, as 
your skills begin to improve.
Try playing a different scale. Start on one of the other fingers of 
your left hand, and play notes up the keyboard until you reach the same 
note on your right hand. Adjust sour notes by using black keys where 
necessary. This is how other scales aside from C major are played. For 
example, the D major scale is played D (left ring finger), E, F-sharp 
(black key), G, A, B, C-sharp (black key), D (right pointer finger).
In addition to learning scales from a teacher or book, it's 
important to keep experimenting with them on your own so that you begin 
to get a head for them as quickly as possible.
Learn about harmony.
Although it doesn't take a lot 
of practice to be able to bang out “Hot Cross Buns” on a piano keyboard,
 unlocking all of your instrument's potential requires learning to play 
multiple notes at once on it, using all your fingers and both your 
hands. To achieve a nice sound with multiple notes, you have to 
understand how harmonies work. This is a very in-depth topic that can't 
be covered in full here, but you can use these notes to help guide your 
practice.
Adjacent notes never harmonize. That doesn't mean they don't have 
their place in certain advanced compositions, but for now, remember: 
playing two notes right next to each other rarely sounds good. Harmonies
 are created by increasing the space between notes.
The space between a harmony's notes is called its “interval.” The 
most common intervals in piano music are fifths, fourths, and thirds. To
 hear examples of these, play C and G, C and F, or C and E, 
respectively.
Harmony intervals go all the way up to the fourteenth interval, 
which is considered a compound interval since it crosses more than one 
octave. Harmonies can also be altered by introducing flat or sharp 
notes, adding support notes, and so on. However, you shouldn't have to 
worry too much about these more advanced styles for a while yet.
Playing the same note at two different octaves is called a “unison” harmony.
Related links :
http://www.instructables.com/id/7-Steps-to-Learn-How-to-Play-Piano/
http://www.true-piano-lessons.com/how-to-play-the-piano.html