Mar
but what should i say.. i wanna be nice about it.. help me! please!!
You might as well go out in style.
Second, the legality of what's going on is an issue here. How long have you been working at the company? At a certain point you are required to give notice - you should find out. Even if you've been at the job for a very short time, it is common courtesy to give them as much notice as possible.
Third, if at all possible, try to help them replace you. Get your shift covered by another worker if you can. Since it's your last shifts, just work them and do the best you can.
Don't forget that everything you do leaves a trail. Once day you may come across these people again and will need a reference (or a potential employer will call them) and you can get screwed.
I would recommend you finish the shifts you are supposed to do, and be as polite as you can when you quit. Don't say "I quit" - it implies someone did something to you (which may be the case, but you don't want them to think so).
Tell them you are turning in your notice. If they ask why, and you don't want to tell them that you hate their guts, tell them that you found another job that can work better with your schedule or some excuse that you can use that wasn't an option at the old place (i.e. more shifts, less shifts, more comfortable shifts, etc.)
Good luck with your new job!
What you said is fine, "Sorry, got a better job offer." Make comforting noises and hang up. Just don't ever use them as a reference.
Good luck!