Zerg is all about economy and imo it's the hardest race to master. With zerg, every larva is either gonna add to your economy or your army, so you have to make a lot of critical decisions. Once your economy kicks in though, you can just swarm the map with units.
Terran is probably the most noob friendly. The gameplay mechanics are simple and marauders/marines are very strong early game units so you won't die to many rushes. They're pretty gimmicky in that they rely a lot on harassment and drops, but they have a pretty strong army when massed up.
Protoss armies can get really powerful, especially late game. Warpgates also help with fast reinforcements for early pushes. They're probably the most brute-force of the three, but aren't that mobile.
I reckon you should play a bit of each race just to see what you like first. If you want pro advice on how to get better check out
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