Actually, tac and engineering would both be viable options to start with.
What I propose, though, would be to go with a Tac in a cruiser or an engineer in an escort to get the best of both worlds.
That way you have a good measure of survivability and damage in either case and it just comes down, which type of ship you prefer.
Escorts are generally smaller, turn a lot faster and have their damage mostly in the frontal arc through preferably cannons like the Defiant or the Akira.
Cruisers are slower and work best with beam weapons broadsiding to dish out slower but steadier damage and have the survivabilty to do so, like every Enterprise you've ever seen (well, the NX-01, which you can buy as a starter ship, is more of a hybrid).
Science captains an ships are much more difficult to master and you should stay away from them while learning the ropes.