Install on your computer a program that allows you to use a regular keyboard as a musical one. Which depends on the operating system installed on the machine. For Windows, for example, PC 73 Virtual Piano Keyboard, KeyMusic. The first one allows you to play both on the physical and virtual keyboard, the second one – only on the physical, but has a more interesting feature: works in the background, and the notes sound even when you type characters in any other program. There is also the Analogx Vpiano program, but it only works with a sound card that has a hardware MIDI synthesizer or its emulator.
In Linux, you can run any of the above programs through the Wine emulator. If you’re not happy with this, use a similar application designed specifically for Linux, such as Vkeybd or Jack Keyboard. Both, like Analogx Vpiano, require a sound card with a hardware MIDI synthesizer or an emulator of such synthesizer (such as Timidity).
To play the virtual piano right in the browser, go to TheVirtualPiano or Virtual Piano Keyboard Online. The applications are made on Flash and are compatible with all platforms where Flash Player is available, including Linux, Windows and Mac OS X. They allow chords to be played by calling each one digital key.
No matter which program you choose, check out her help with matching computer keyboard keys to notes. There is no standard here. If the program allows you to reassign keys, use this function if you want to select a convenient location of notes on the keyboard.