How do I treat this spider bite? What do I have to put on it?

First, avoid infect. A spider bite can cause a large amount of redness and swelling, and since the bite causes a break in the skin barrier, it could leave exposure to strep or staph or skin organism to get onto the soft tissues causing infection (cellulitis). So first keep area very clean. Then i suggest a mild Hydrocortisone cream 0.5% (otc), alternating with an antibiotic cream. Don't scratch!