There is no best race. However, most beginners start with Terran since they're familiar with the race due to the single-player. Probably the easiest race to play is Protoss, since you can literally 4-gate every game and have a good chance of winning. Zerg is by-far the hardest race due to all the extra mechanics involved (injecting, scouting with overlords, spreading creep tumors) as well as needing the gamesense of when to make drones and when to make units.