As a kid that just came out of High School, is it a good idea to go straight into the military, or should I wait and keep training to get stronger/faster?

It depends on the person, but in general, I rarely see 18-year-olds make it through and get selected. I would say mostly because of their lack of maturity in adverse situations. That being said, I would recommend you take care of anything you need to in order to be able to make your career in the Air Force your main goal. Then come in.