Take the entire car apart piece by piece making sure to label everything. Once that is complete, you can then determine what can repaired , what needs to be replaced and what is missing. Make an inventory list of everything as you go.
Next, determine what your budget is going to be, and be realistic. Saying you have a $20K budget but no way of attaining that level is going to cause you to have a torn apart car for years to come. Remember that it is cheaper to reuse the parts you have if possible. Do everything you can to find them second hand. eBay,a Craigslist, swap meets are all excellent sources of used parts.
Keep in mind that your project car is a 1973 mustang
. In the big scheme of things, it is not the most popular model/body style. Used parts are going to be harder to find and the after market is not nearly as big as say, a 1966.
Have fun, be realistic with yourself and your capabilities. It is fun to learn to do things on your own, but know where your limits are.