That is tricky if you don't know what an altcoin is I'm not sure you will have fun getting to know how to mine them. Anyway- an altcoin is an alternative coin to Bitcoin.

You can find many new and old altcoin here https://bitcointalk.org/index.php?board=159.0 So in short you would be mining an alternative to bitcoin which you can more than likely find help with from here https://bitcointalk.org/index.php?board=160.0