So as a frame of reference I know little more about cars than how to start it, drive it and fill it with gas.

I bought a new car in Dec 2013, and I hate it. My car had been totaled and I had to make a quick decision on a new car and in hindsight I made a bad choice for me. I want to ditch this car and get a new car, but don't want to be completely stupid and wasteful with my money. What's the best time to sell it to maximize the value I'll get for it, versus the value I will lose for every extra mile I put on it? It's got 18k on it so far. My mom says, start looking for a new car when I hit 40k and sell before I hit 50k, not sure if that's good guidance or not.