If you have never ridden before a friends dirt bike is the way to go. 125 or smaller.
Falling over in dirt doesn't hurt (going at learning speed anyway) and depending on terrain won't hurt bike too much.
You will want helmet, gloves, boots and a jacket.
If you do have a friend that has a dirt bike he will probably let you use his equipment so all it would cost is
gas and some beer.
If you don't have a friend with a dirt bike I would suggest taking a riders course.
Around here it costs 200 bucks.
What does 200 bucks get ya?
They let you use their bikes (which are donated) so you don't