The classes do play very differently, and only become more varied as the games progress. If you're confident in shooting, pick soldier or a soldier-aligned class, like vanguard (close range) or Infiltrator (long range). Defensively, the sentinel is your best bet, and has the best of both worlds though in the first game their firepower is severely lacking, while Adept is essentially the mage of this series. The engineer sort of is as well, but with more tactics; they get a sentry drone in later games.