Assuming that you already have basic knowledge of playing the piano, and are learning how to sight read the music sheet a few hints on how to learn to read might help. Just like in typing, playing the piano requires that you do not look down at your hands, and that reading the notes becomes as second nature as reading words in a book!
Find music books at your local library, borrow books or print free sheet music from a website (see recommendations below) at your skill level or easier. This should be music that you think you would like but have never heard before.
Sit at the piano, and open the book to the first page. Try to look at the notes, say the notes softly, and understand the piece a little bit without actually playing any part of it. Look at the key signature, any changes of clefs, and the dynamics of the piece. If you can, look for chords, and try to determine what they are. Look for the trickiest part of the piece, for example, semiquavers, or a corner with a lot of accidentals that are difficult to digest, and determine a speed at which you think you can play even the trickiest parts. It is very important to not stop and restart when you make a mistake. Just keep playing. Look for patterns while you are playing, and always try to read a minimum of one measure ahead.
When you finish one piece, repeat it until you feel comfortable knowing the names of the notes. Remember, it is like typing -- your mind will tell your fingers where to go once you practice it often enough.
Continue with the same piece, and when the notes are easy to play, learn the timing of the piece. No piece can sound decent if the timing is incorrect.
Keep practicing in this manner as often as you can. Feel free to go back and study the pieces you played in more depth. The more you practice, the better your sight reading skills will become.
You can, if you do not have a piano nearby, read the music notes without even playing. Look at their positions and remember what they look like. Get it to memory.
It is recommended that you use an program like Fast Keys (a small game that greatly speeds up the learning process of reading music).
A very good skill when sight reading is recovering from mistakes. You will make them. Don't let them fluster you; just keep playing. It is almost guaranteed that if the listener isn't familiar with the music, and you don't give it away, they'll never know.
One of the hardest parts of sight reading music is getting the rhythm correct. It helps to count out loud, "One and two and three and four and..." Of course the numbers that you count depends on the piece.
It helps to accompany a singer or other instrument while sight reading. This forces you to stay honest.
Check for sharps or flats, key changes, or changes in the time signature. If you are dealing with any large jumps (i.e., octave jumps), also beware of those. Double and triple check any note that isn't in the staff.
Do not judge yourself or your ability as you sight read. Remember, you are simply trying to improve your reading skills. Since sight reading involves playing continuously (not stopping as if you were practicing to perfect the piece), your focus is important. Getting angry at yourself or putting yourself down to any degree merely distracts you from the main goal. Smile and play with intent.
Another very good (And far more enjoyable) way to practice sight reading is to play duets with a friend, as both pianists will be forced to keep in time, play continuously and with correct notes as far as possible.