How do i go about fostering a guinea pig, and what do i need?

The basics for any guinea pig.. food, water, bedding (ceder usually), & cage. Maybe some Timothy hay & an igloo if you can. Plus fresh fruits and veggies as snacks.

A cage. You can make a 2x3 c&c cage
http://www.guineapigcages.com/
Bedding. Pine and cedar are toxic. You can use aspen, carefresh, other paper based beddings or fleece
Hay, high quality pellets and veggies. This talks about diet
http://www.guinealynx.info/diet.html
A water bottle, food bowl, nail clipper, a place to hide and toys.

Im not sure but I think you would need to find a guinea pig rescue and see if you can foster.