If i have an abscess tooth and take penicillin will the swelling go down fast?

Not necessarily: Antibiotics, in conjunction with the start of root canal therapy is the best way to get rid of the infection fast. Antibiotics alone don't always do the trick, and you may need a different one for your infection. See a local dentist for treatment, since the antibiotics will not fix the tooth and it will become reinfected very quickly.

Maybe: Depends on the type and location of the abscess. It is best to have definitive treatment performed as soon as possible -- root canal treatment, if the tooth has a nerve abscess and is savable or extraction if not savable or periodontal gum / bone treatment if you have a periodontal (gum) abscess.