I have computer where I have only CPU and don't need to pay for energy. What kind of virtual currency should I mine?
Here you can check profitability of all the algorythms and many currencies - CoinWarz. Possibilities of your computer you have to check by yourself, by installing mining software and running it with wallet or conneting to crypto-mines.
You can register an account with a multipool. This is a pool that automatically switches to the most profitable coin.
Because you are using a CPU, the only viable option would be a multipool that also (or only) mines X11 algorithm coins. This way you'll at least get something. (Mining SHA-256 coins like Bitcoin with a CPU will get you less than 40 cents after a YEAR of mining.) Both multipool.us and coinmine.pw are multipools that mine X11 next to other algorithms.