Before you do anything else, please take your machine to a Apple Store, you have 3 free months of care and they will assist you, perhaps you can take some classes how things work.
Tell them you only had the computer for two weeks and the startup disk says it full, and you've deleted some items, they will know what to do.
Your startup disk shouldn't have gotten so full so fast, something is clearly wrong and you need help in person, we can't assist you here I'm afraid as you don't know very much at this point.
Ask about getting AppleCare and a TimeMachine drive, they will fix your machine right up.
Good Luck Katie.