Strep throat refers to an infection caused by bacteria called Group A Streptococcus pyogenes . But those symptoms don't always mean you have strep - they accompany many viral infections. The only way to really know if you have strep is to visit a doctor and get a culture taken. There are also some very rapid tests that give results within minutes, but these come up with a high percentage of false negatives.
Standard cultures can take several days.
Many doctors will make a tentative diagnosis based on their observations and begin treatment pending the results from a culture. Penicillin is the standard treatment to kill the bacteria. Some doctors use other antibiotics, but they're more expensive and less specific - adding to today's problem of antibiotic resistance. Treatment is also begun right away because the symptoms can be so painful. Generally, you should feel some improvement within 24 to 48 hours after starting the antibiotics.