You might not need any kit of parts to go singlespeed, depending on what bike you have that you're goiing to convert.

Gearing is a personal preference based on the muscles in your legs and what you mean by dirt jumping. If you already go dirt jumping then you must have a favorite gear you use already. 32x16 or a 2:1 ratio is supposedly the favorite dirt ratio but jumping might mean something else.

I use this chart alot to decide about gearing:

http://www.fixedgeargallery.com/resources/gearchart.jpg

Master Links, your bike shop will have them and tell you how to use them and sell you a chain tool as well. sorry don't have a weblink handy.

good luck.