Credit available/balances, how does it work?

So I just got my first credit card and sadly got a little excited and maxed my limit basically. (Close to it at least) well... my limit and available credit was $300 and i now have a balance. Now what i want to know is...if I want to be able to use the card again by paying it off do I have to pay the whole amount monthly or do I only pay part of the balance? And will my available credit go back to $300 or just what i paid back so far? what depicts how much is available if this makes sense? Credit cards and numbers confuse me.