If you don't have a keyboard at home, you can use your computer as a 'virtual keyboard.' (You'll need to have your speakers on) These virtual keyboards are useful if you just need to play a few notes that you're not quite getting right. It's a very quick and efficient way to work on a tricky section of a song. There are a number of versions on the net.
All have their advantages and disadvantages depending on what you're wanting to play.

Here is a selection: you may find more by simply by googling 'virtual piano' in your browser

Virtual Piano

Advantages: Large keyboard
Disadvantages - notes sustain so you have to play slowly or notes will 'clash' with each other.
Notes are not named (Download and print this handout and it will tell you the names)

Virtual keyboard

One of the simplest virtual keyboards on the net.
Advantages: each note has it's name written on it.
Disadvantages: it only shows an octave and a bit (meaning its range is quite small)

Online pianist

Advantages: Nice large keyboard. Notes don't sustain too long
Disadvantages: no note names
(Download and print this handout and it will tell you the names)