So for the past day, I have been interested in Cryptocurrency

What I did so far, got BTC and waiting for it to deposit into Mintpal for trading. What I don't understand are the boxes for buying/selling Altcoins? Which box do I fill in "Amount" or Price per XC"? Can some someone explain to me the whole logic? I just mainly know is Buy cheap and sell high. Sorry if this has been asked already, had trouble finding a tutorial for Mintpal and trading etc.