Gosh, DustyTrail01, sounds like you are really pumped about driving. It is good to hear such enthusiasm.
BUT, listen to what CheyAut said....
Driving is sometimes very dangerous. Unless your grandpa is some sort of mechanical engineer, I wouldn't trust a homemade cart. Even professionally made carts can break apart, turn over, and you can get severely hurt. Carts must be made to be perfectly balanced for the comfort of the horse and the safety of the driver. Dimensions must fit the horse perfectly well.
Same for harnesses. They must be made so they are comfortable to the horse and must fit. Even well fitted harnesses are frequently adjusted during training and driving. If made out of the wrong materials, you could have a disaster on your hands.
You can buy a sturdy easy entry cart and a beginner harness made by professionals who really know what they are doing. I would really suggest you starting with that.
Driving is so much fun, but there is no reason to take a chance with your safety or your horse's safety.
There is probably someone in your area who can advise you. I have been driving for over a year and I still have a lesson from a trainer every week. I want to learn and I want to try to be the best at what I'm trying to do. After a year, I still have SO MUCH to learn.