My boys have built many over their years in cubs and scouts. Here's some pics of a typical cub car.
The most important things to watch are making sure your car is at its max weight (important to speed) then putting that weight as far back as possible but not too far that the front wheels want to pop the car off track. Secondly, make sure the wheels roll freely and are properly aligned to go straight down the track and not making the car scrub the side of the track, slowing it on the way down.
Good luck with your racer.
P.S. I have a track to donate to a cub or scout group if you want to practice/tweek/cheat/pre-race/test/tune.