The vet may just drain it, and apply antiseptic, and probably give you some to keep bathing it with (it will be important to keep the wound open until it is fully drained) Or, like Misfits4Me said, a course of oral antibiotic may be necessary.
My dog had an abscess
on her back a while ago, and the vet drained it and washed it out with antiseptic, which I then had to do 3 times a day and allow the wound full drainage to stop it filling up again.