While there are lots of good beginner corals, id suggest you do some research on what you like first. A lot of people will recommend Green Star Polyps or Xenia as a great beginner coral. And it is. its bullet proof. But its also invasive and will take over. Like another user suggested, i'd start off with some candy canes (they come in lots of colors), maybe a toadstool, or some zoanthids or palys. Make sure when you get zoanthids or palys that you get the varieties that aren't invasive. Generally the ones to stay away from are the nuclear green palys, common brown button polyps, yellow palys. They grow fast, and you'll be tired of them when you get more colorful corals.
Once you want to get into more intermediate corals i'd suggest favias/favites, some euphyllia like frogspawns, and hammers, and maybe even some beginner SPS like monti caps, and birdsnests.
Good luck!