"I kissed her and found out she had strep throat. Now what do I do?"
Strep throat is very contagious following direct contact. The incubation time is 2 to 4 days after exposure. You are probably best to get a strep throat throat swab test at a nearby clinic. The rapid test is pretty accurate after 48 hours after exposure.

"Should I go on antibiotics if I get a sore throat?"
Testing is very important since there are many causes for sore throats including; viruses, post nasal drip, allergies and acid reflux none of which are cured by antibiotics.

"I took a couple of my room mate's antibiotics for my sore throat and it felt better for awhile but now the sore throat is back and I don't have any more antibiotics."
This is a common problem since taking antibiotics messes up any chance of making an accurate diagnosis and leads to unnecessary antibiotic use, bacterial resistance and occasionally very severe complications for otherwise minor infections. Never take antibiotics that are not prescribed to you and if you do have a prescription take the whole course.

"After I start taking antibiotics, how long am I contageous for?"
You should avoid close contact for 24 hours after beginning your antibiotics.