How do you treat your foot that got punctured by a rusty pin?

While rust actually carries no dangers in and of itself, tetanus passes through fecal matter, which is often found on the ground. So, anything that creates a wound -- especially a puncture wound (which can close over and make a little pocket of infection) that was on the ground, means you need to be current on your tetanus vaccine.

You need to renew your tetanus vaccination at least every 10 years. If you're current, clean out the wound as best you can with running water, then with soap and water. Hit it with an antiseptic like hydrogen peroxide, and bandage. Wound-check it (i.e., un-bandage the wound, look it over, then rewrap with a clean bandage) the next day. If you aren't current on your tetanus vaccination, you need to get one ASAP.