The more money you get with the type of job that you do in the garage. I started with the simple wash and dice for 3-4 levels (dont recomend that after3) and then I started doing wheels every 4 hours. As you go in the game the jobs value to level up get shorter and shorter but the cash will stay the same. When you start the game, get the pinto and max it out and that is one of the fastest to start with. I also have an NSX and I plan on getting the R34 GTR when I hit level 23. I saved up 157k to get the place to where it is now. you have to be level 18 or 20 to get the 25x25 floor expansion. Each job has a set time and after the time expires you have the same amount of time to retrieve your money or else the customer will leave without paying.
When you bring in someone onto the lift, select the car and then hit stop and then you get to choose the jobs. Also when the lift is empty you can select it and then choose a job.