Decide what type of music you would like to play. Free sheet music is available for many classical, pop and rock songs, and you can search for a particular song or for a genre.
Search on the Internet for "free keyboard sheet music" and the name of the song or the genre of music you would like to play. The search will bring up hundreds of results, and while you sometimes have to click through several pages to get to the actual music, you can download PDFs without much trouble.
Print out the sheet music. It will be much easier to have the music on paper rather than trying to look at a computer screen while learning to play.
Study the sheet music while sitting at the keyboard. Standard musical notation consists of two staffs of five horizontal lines each. The upper staff is called the treble staff and is played with the right hand. The lower is the bass staff, which is played using the left hand. The time signature is the two numbers on top of each other on each staff at the beginning of a piece, such as 4/4 or 3/4. This tells you how many beats per measure, measures being denoted by vertical lines through both staffs. Music written in 4/4 time has four quarter notes per measure, while in 3/4 time there are three quarter notes per measure.
Place your hands on the keyboard for the initial notes. Notes are denoted by ovals which can be filled or not, and with or without tails (short vertical lines which go up or down). Their position on the staff tells you what key to press. In the treble staff, the lowest line corresponds to the E above middle C on the keyboard. The space above that is F, the second line is G, the space above that is A, the third line is B, and so on. On the bass staff, the highest line is A, the space below that is G, the line below that is F, and so on. Sometimes there are sharps (represented by number signs) or flats (represented by a symbol resembling a small letter "b"). If the sharps or flats are at the beginning of every staff, it means that every time that note is played, it should be a half step higher (for a sharp) or lower (for a flat) than the note. For instance, for a song written in F sharp, every time there is an F note, you would actually play the black key just above and to the right of the F key. Occasionally, there will be a sharp or flat symbol in the middle of the music which only refers to the note which directly follows it.
Play the initial note or notes by pressing down on the corresponding keys. If there is more than one note on the same vertical line at the beginning of a piece, all are played at once. To determine how long to play the notes, you must pay attention to what the individual notes look like. Whole notes, empty ovals without tails, are held for four beats. Half notes, empty ovals with tails, are held for two beats. Quarter notes, filled ovals with tails, are held for one beat. There are shorter notes as well: an eighth note is a filled oval with a tail and a flag, or short diagonal line off the end of the tail. The flag may be connected to another eighth note, or it may be free-standing, but either way, the note should be held for half a beat.
Count out loud or to yourself as you hold down the keys. How you count is determined by the time signature. For a piece written in 4/4 time, you would count "One two three four, one two three four," with each number representing one beat, and each repetition representing one measure. For a song in 3/4 time, you would count "One two three, one two three." This way you will know how long to hold each note.
Continue playing, following the sheet music from left to right, counting out the lengths of the notes as you go.