What is the best way to treat a spider bite?

It depends on type. Of spider. Wash area of bite with warm water ; soap. Dry. Apply antibiotic ointments. But seek medical care asap if you think a spider bit you ; you have fever, severe pain (can be felt at bite site ; possibly chest, back or abdomen); abdominal cramping /rigidness or ulceration of skin at site. You may need a tetanus shot (if you don't know when your last shot was or it has been > 10 years). Also.
Find spider. Most spider bites are not serious and supportive care is all that is needed. The spiders that cause serious envenomations need to be examined eg. Black widow and brown recluse spider. T.