Here's a quick rundown:
1. A truck (the newer/more reliable the better)
2. A plow (anywhere from $2000-$4000 depending on model)
3. Insurance. You need to have plowing insurance on the truck, and it's also a VERY GOOD IDEA to have business liability. Usual coverage is $1,000,000/$2,000,000
4. If you want to spread salt, you'll need a spreader. If you're gonna sand, you need a spreader capable of spreading sand. Costs start around $1000 and go up from there.
5. It's a very good idea to have some sort of warning light on the truck. Some prefer revolving lights, others prefer strobes, some have both. Don't get me wrong, people will still cut you off and get in your way, but at least you warned em...
6. Extra cash in case of a breakdown.
These are basics. I'm sure someone else in here will fill in anything I forgot.
Good luck!
-Tim