How to tune a freshly built engine?

As far as I know the computer of the car decides how much output your engine has based on the air injection, fuel injection, timing, etc.
So if you build an engine for high power output how do you have it run for what you built it for?
Let's say I take a Ford 5.8L Windsor. I put aftermarket heads, headers, exhaust, cams, forged Pistons, rods, rings, cams, intake, and all that and make it in a way that it can handle 300-400 hp while stock it puts out only 215.
It wouldn't work if I put it back into the car without having the computer provide the engine with what it needs to put out the power it can.
So what procedure would I have to take?

Does anyone in the Georgia area have a car they want to get rid of?

I'm in desperate need of a car. I'm a 18-year-old girl. I sometimes borrow my grandmother's car, but now that she has a job it conflicts with my schedule; she also pics of a family friend's child from school around 330 every day. I have a part-time job, I also am A aspiring actress, but since I don't have a car I miss out on different gigs and also auditions. That's the career I'm trying to pursue so if I miss out on opportunities they won't be any and I don't want to be stuck at my part-time job forever or just go to school for something I don't want to do. I really need a car just to get to regular places like my acting classes but I can't a Ford a monthly payment and I also don't have credit for even a cheaper but used car. So the main question here is there anybody in the Georgia area who has a car they no longer need or would like to get rid of? It would be great for me. I really don't don't know what other options there's for me and public transportation is a no-no or is hard to use in Georgia.