What's the best way to get a thirteen year old girl to pee after a bought with dehydration?

If she is properly : Rehydrated, she will urinate normally. If she doesn't, there may be another problem, like a kidney injury. Have her evaluated by the er if that's the case.

More fluids: Please increase her fluid intake and have her avoid caffeine. Give her water, gatorade...Whatever she'd like to drink.

If it hurts: A good way to get her started is to let her sit in warm water in a tub and let her pee.The warm water helps relax the pelvic muscles and an episode or 2 in the tub will allow her to pee without discomfort . She is then likely to resume normal activity in voiding.