As with other Horizon games, you get experience points in a variety of ways. Winning races, Showcases, PR stunts and Bucket lists mostly. Plus you get extra experience from smashing XP boards, discovering roads, scenic areas, etc. As you get experience, you will start to level up. At first very quickly and over time more slowly.
You can't miss this one unless you just quit playing the game before you get to level 20. You should easily be able to get to level 20 in the first few hours of playing as long as you are not just exploring the map the whole time.