I have a 1999 Pontiac Sunfire and the hood is starting to get rusty. also, the bottom 'rocker panel' underneath the doors are getting rusty too. This might be a dumb question, but is there any stuff i can buy (that would work well) to stop the rust from spreading? and if so, what is the best way (aka cheapest way) to repair it?

If cosmetics aren't important, or if it's not visible, I've used a rust preventative paint called POR-15. Go to Ebay and search for POR-15. They sell a "Super Starter Kit" for about $20 that comes with a cleaning solution and pre-treating solution and a small can of the paint. Only comes in a few colors, but used correctly it seems to stop rust cold.

Jegs sales some stuff now for that as well. Now the proper way to fix it is to cut it out and weld in new metal. Don't half ass it and use bondo.