Race

I just moved to higher race class, now I'm losing every match, how can I upgrade my car if I keep on losing matches I'll get no points to upgrade..

You generally want to save up the points from your previous league to your current leage so you have slight edge over others (or in this case aren't as much disadvantaged). So when you're at the class E you don't spend any points on upgrades, then on D class spend those 25 points from E class and start saving again to again use all those saved up points from D to use in C and so on.

If you've already wasted your racing points there's not much you can do outside of keep racing and you'll get a lucky win here or there until you accumulate enough points.

Other than that you can also join mechanic shop to exchange job points for racing points.

Managed to dig some guides on this. This one goes more into detail on how to get unstuck from a class with no points.

This one tells you what to ideally upgrade for the best results. Just remember both of these are bit old though the only actually outdated thing I found from a quick skim is just that gas station no longer provides racing points and that the names haven't been updated. You can check which car they mean under the new naming system here.

Eventually you will get enough racing experience to win now and then.  What I did in the lower classes was when I started to win a few tracks I stopped upgrading. 

If you plan on racing long term everything until b is just grinding skills as the cars won't be used anymore.  The idea is to spend as few points as you can to progress to B class where you buy the first car you can use in A.