Can I buy worm medicine for my puppy there? And how old do she have to be to get medicine?

Yes

yes and 6 months atleast to be on safe side

You start medication for worms at 2 weeks and continue every 2 weeks until about 12 weeks.. afterwards treat as needed or when signs of worms appear... Or you can treat about every 6 months as a preventative. Follow dosage directions included with medication.

Yes, check with a vet. It could depend on weight.