My daughter thinks she has urinary retention and told me she has felt the constant need to pee for a few months. should she do kegel exercises to fix this?

Pelvic floor issues in a girl her age would be WAY down my list of possible issues. Far more likely would be a simple UTI but other more dangerous possibilities exist such as diabetes. You also have not mentioned if she's started her periods yet. Take her to a pediatrician, sooner rather than later.

your daughter's presumption might be wrong urinary retention is very rare in 13 yr. old girls. She needs to see a Urologist and have her urine tested too. Kegels are not indicated in her case