Accidentally accepted job.

I recently applied to lots of jobs because I need one in order to get a car and help pay for my college tuition. I’m a 17 year old dual enrolled student and haven’t had a job in a year now… I had a video interview and then a phone interview with a company that seems really nice a few days ago and I got the job. I have training shifts coming up this next week. Today I saw that the place that I’ve been trying to get a job at for over a year now wants an interview with me… I don’t know what to do. I have the interview planned early next week. See, I would tell the company I’m working for now that I sadly can’t continue… but I don’t even know if I’ll get the other job! I don’t want to work for them for a few days and then ditch them… I also don’t think I can work two jobs while still maintaining my 4.0 GPA in college! What should I do if I do get the other job? How do I resign in a respectful manner? Thanks! Both of the jobs are part time and sales associate/cashier positions if that helps.