I have to buy a gun...

[Fleshbits]

So, in the tutorial we were given guns, but then they took that ship away. So, I am running career agent missions. This fellow says I am going to meet hostiles. I imagine I need a gun.

What kind of gun do I buy?
I assume they eat up energy, so you have to have a capicitor able to feed them?

The ship I am currently flying is a Heron with 281.75 GJ

[Samus Aran]

a heron can be fitted with rocket launchers and 75mm rail guns.

[ISD Max Trix]

Different weapons use different amounts of capacitor. It really depends on what race your are (If your an Alpha) and the ship you are using. I suggest asking for help in the IN GAME ROOKIE HELP Channel for assistance. You can also try the official forums New Player Q&A

[battlecattle]

Sounds like you're caldari, in which case you'll want either a kestrel or merlin frigate for combat.

Kestrel is a missile boat which can use either light missile launchers (longer range, less damage) or rockets (short range, more damage). Missile launchers do not use capacitor to activate. For basic ammo, you'll want to select the missile you use based on what damage type a target is weak to (for instance, gurista npcs are weak to kinetic damage so you'd use scourge missiles). If a ship has a bonus to missiles of a particular damage type, use the missile of that damage type (for instance, the drake battlecruiser has a bonus to kinetic damage, so one would want to use scourge missiles with the drake).

Merlin is a hybrid gun boat which can use either rail guns (longer range, less damage, slower tracking) or blasters (short range, more damage, faster tracking). Hybrid guns do use capacitor to activate. Hybrid guns always do thermal/kinetic damage. One generally selects the ammo one uses based on the range of one's opponent, shorter range ammo doing more damage.