stop worrying about things that don't matter

Nowadays I worry about every little minor thing, I worry that I might be late for school or walk into the wrong class, I’ve worried about whether I was going to get called on the way to the car again (because that happened once). It’s really annoying because I seem to have an innability to think rationaly, but that’s all going to change – I’m going to come up with a system to stop me worrying! Does anyone have any ideas?