If your happy with your current gearing just count the teeth on tyr large and small cassette cogs and buy the same, eg: 11:25 or 12:28.
Fitting is easy, you just need a chain whip and a cassette tool.
YouTube has how to videos as well.
Don't forget you will probably need a new chain as well, (it's good practice regadless), and depending on the rate of wear possibly chain rings as well.