You need to finish on the top 3 for each the first three sets of races to unlock the 100 HP Championship. Then, you need to be on the top 3 overall ranking for that championship for the achievement to unlock. This first championship is the easiest one and you'll probably won't have any problems winning, but if you do, my advice is go to time trial and learn the tracks (shortcuts, shorter detours, tricky curves or jumps, whichever works best for you) & become familiar with the car you'll be using to race (faster cars are usually more unstable & you end up crashing a lot or maybe land facing backwards, so you may do better with a slower car that's more manageable, like the truck category).