I'd start off with some basic repairs/upgrades and learn how to do those first. Replace the brake shoes. Make sure the chain functions properly--smoothly, with no kinks. Learn how to measure the chain while you're at it. Change out the brake and derailleur cables--both housing and inner cables. Clean up and polish the bike so you can see what you're doing. Remove any rust everywhere you can. Change the inner tubes and maybe even the tires.

I'd get all these basics down before moving on to wheel axles, headsets and bottom brackets.