Look for sites that offer virtual keyboards like bgfl.org or pianoworld.com. Check out a few different websites and see which ones seem the most interesting to you. Some sites offer more than just piano sounds, and will allow you to change the tempo of your rhythm track.
Read the instructions on the website that tell you how to play each individual keyboard. Most sites are laid out so that the A key is middle C, the S key is D and so on. Some websites have chord functions, which allow you to play only one key while the other notes that make up the chord are added by the computer.
Click the virtual piano keys on the screen with your mouse or use the keys on your keyboard. Try to play a scale -- A-B-C-D-E-F-G -- in succession to practice using the keyboard.
On many virtual keyboards, you can choose to have a rhythm track play with you. Many kids find this to be a fun activity, and it can help both children and adults learn to recognize notes, rhythm, and their relationships to each other.
Try to play a simple song like "Mary Had A Little Lamb" or "Twinkle Twinkle Little Star." If you do not know these songs, you can find the notes online at sites like musicnotes.com. Once you get the basics down, try to write your own songs.