I'm a teen. How will having pyelonephritis mess up my health later in life?

Depends on situation. For some, pyelo is a one time infection that responds to treatment and the patient goes on with a regular life. For many, pyelo is one of the complications of other health issues, like spina bifida, renal tract abnormalities, systemic illness, etc. When part of other issues, it can be recurrent and eventually lead to kidney failure. If so, dialysis or transplant may be in the future. .
See below. Should not generally leave any sequela, if no cause is found however if it is due to a cause like a kidney stone or any defect in the kidney structure, it may be recurrent.