What to do for a sider bite?

Wash it with soap and water! I didn't and bacteria moved in the large hole behind my knee where it bit me, and cellulitis soon covered my whole leg.
If you saw the spider, look for pictures online. Where I live (northeast US) there are no poisonous spiders so I didn't get treatment until cellulitis started. If I had kept it clean, it would have been uneventful.
Spider bites often swell up several inches wide and 1/2" high. It doesn't mean you have to rush to the ER unless it was poisonous or you don't know.