Please note: Here is a submenu, too! (Daily Races, Championships, Tips & Tricks)
There are several types of competitions in Star Stable. The most popular form is the race, but there are Pole Bending, Barrel Racing, and Show Jumping.
When you have accepted to compete in a race you can't start right away. Wait for the countdown and when it reaches 0, ride as fast as you can. You have to pass all the checkpoints along the way to succeed, and finish by crossing the goal line. Checkpoints are marked with flags on the sides and a big green arrow in the middle.
After you've reached the goal you can compare your result against the other players on the high score list. There is a separate high score list for each race.
Use the buttons in the high score window filter the results. You can also check the "List only friends" box to only see yourself and players on your friend-list.
When there are two or more players in your group (max five people) you can click on the race starter notice board. If everyone in the group is close to the same starting point, they can accept the race or leave the group by clicking on the square under the group information.
When everyone in the group have accepted the race it will start and the members will compete individually.