Good advice in this thread.
What I'll add is that if you can pay it in cash, then you should. My rule of thumb is for things like groceries that amount to $40 or less or lunch for $10-15 (presuming they take credit cards), I prefer to pay cash. If say, you're treating 4 of your friends to a nice restaurant ($50 or over), then use your credit card, as long as it's within your means to pay in full the next month. I remember when I got my first AMEX, I spent $700 on the trot with no interest, but had to pay it in full the next month and if I didn't have enough to cover, I would have freaked out.
In your case, paying off small expenses with a card is fine to build credit. If you're splurging a bit, need to buy a big ticket item (i.e. laptop), or have to spend it on emergency, make sure you can cover minimum payments with that card. Then, in due time, if you've saved a bit, keep paying more than the minimum payment so that you don't get hit hard by the interest, no matter how low/high it is.
It's kinda like the Old Cafe - http://fbc2.freeforums.net