Start by doing some research and putting together a plan. Do some searching here on what others have done. Brakes, steering, suspension, differential, engine are the big ones. And these affect each other, which is why planning is so important. That way you don't end up with 5-bolt Ford wheels on the back and 6-bolt Chevy wheels on the front.
Also go to some car/truck shows. Talk to the owners. Most will gladly share their knowledge. There is a local club in KC.
www.chevygmcofkc.org
And some parts vendors. Lil Red 66, Carters, Classic Chevy come to mind.
TR65 gave some good advice on working on safety issues first. Brakes, seat belts, wipers can usually be done for not a lot of money. Engine tune-up will usually help the mileage. Work on the small items while you are working on your plan.