[Signman]

To begin go to the track to spectate walk the pits stop and talk to racers befriend a few over time.
If possible get involved with a car. What you will learn is invaluable will speed up the learning curve many years.

Pick the brand you prefer, pick a few bodies, years and engines you like. Study the Classification Guide and Engine Blueprint Specs.
Ask questions and look over these cars/combinations at the track. Be a sponge not a brain picker. Subscribe to Nitro Joe's Stats.

Pick a car you like or love because this takes a lot more time, work and money than you can ever estimate.
There are some benefits to just buying an existing race car so don't rule it out if not up to the monumental effort it takes to build a car from scratch.

As far as how fast it needs to be on the qualifying sheet is pretty much defined by the combination and your wallet plus work ethic: Wallet the most important.

You have to know why your doing this:
If you want to win races the car needs to be able to run 1 second under the index at tracks you will compete to be a threat in heads up races AND be consistent AND you have to be able to drive both ends of the track. You don't have to run the car a second under just be able to. The car must be a weapon and the driver has to know how to use it.
If you want to go fast meaning 10 seconds and under and qualify on the top of the sheet it takes deep pockets if you got it go for it. These faster cars tend to be more consistent and more easy to drive in the eliminator for several reasons including you'll chase a lot.
If you don't have the buck$ and just want to be part of the scene and have fun disregard the above buy a car you like and enjoy it.

Good Luck!