How much is it to fix a head gasket on a 2000 ford expedition

A 15 year old car! Not going to be worth much as a classic in 10 more years. Unless the body/interior are showroom, I would scrap it for a few hundred dollars and buy another car, a little newer. If that is not an option, then pick up a used engine for about $2000. If it was mine, I would junk it and get another one but newer. I doubt you could get more than $2200 for it if it was running. Not what you wanted to hear but that is the practical answer.

My truck is nice. And I need to know what I need to do to do this job myself?

Unfortunately, if you don't know what tools and parts you need to do this job, I am afraid you would not be able to do it yourself. You are dealing with aluminum heads so you will need to have a machine shop available also. Good luck if you want to try to do this yourself.