Leaving job for new job

So this looks really bad, but I'll try and explain. Recently I was hired for a part time job that pays 9.75 per hour. Upon hire, I was under the impression that I would be getting full time hours. Sadly, I was mistaken. Before accepting this job offer, I had also applied and interviewed for a position with a different company as well. So the first company I currently work at hired me first..I've been there about 3 weeks now. Now the other place I had applied offered me a position (it took them longer to get my background check back) and this position IS full time with benefits so I would rather work there...they want me to start asap so I told my current supervisor. They haven't made the schedule yet for the next two weeks, so my supervisor said ideally he'd like a two week notice but if I can finish out the next two days he should be able to work it out.....BUT I don't want to burn any bridges. What should I do?! He said I can still type up and email him the notice tonight.....can anyone help me with what I should say in the letter please?!