First, in your character sheet, the skills in the Engineering group are good for making all your ships fit more/bigger things. Energy Management, CPU Management, train these skills to 4.
Second, fitting a ship:
2a. Read the ship description text to see what it is intended for, what weapons it has bonuses for.
2b. Fit those weapons. With every weapon you have 2 choices: Long Range (125mm or 150mm rail gun) or Short Range (light ion, electron, or neutron blaster).
2c. Frigates MUST have speed. Put in a 1mn Afterburner to give yourself speed. For PVP, look at 5mn microwarpdrive.
2d. Fit defenses. Gallente typically use armor defenses, and frigates don’t have enough fitting slots for a full set of plates + armor resistances, so just fit a small armor repairer and a damage control and call it “defended”.
2e. Fit damage enhancements. If the ship uses guns, look at magnetic field stabilizers, if the ship uses drones, look at drone damage amplifiers.
2f. Fit utility: If you’re going to be using short range guns, a stasis webifier is nice. For missiles, a target painter is nice. Otherwise, capacitor rechargers or capacitor boosters are nice.
2g Fit your rig slots. Frigates use small rigs (and you need to train the rigging skills to fit rigs). Look at capacitor recharge rigs, drone damage rigs, basically, utility rigs.
Tristan is a drone ship. It can fit 2 guns, but the main damage will come from the full set of light scout drones that you can control during the fight. Make sure that the Drones skill is trained to 5, so you can control 5 drones. You can put some light blasters on it for extra damage against the tougher opponents that the drones can’t overcome by themselves. Defenses are 1mn afterburner, small armor repairer, drone damage amplifier x2.
Incursus is a gun ship, it can fit 3 guns and only 1 drone. Make sure your Gunnery skills are trained, and a typical fit for the Incursus uses 3 (short range) light ion blasters with antimatter or uranium charge S ammo (antimatter is short range ammo), 1mn afterburner and stasis webifier to keep the target close as you shoot it, and the typical small armor repairer, and 2 magnetic field stabilizers in low slots.
Incursus and Tristan are both combat frigates, and they’re pretty good at it. One is not better than the other, it’s just a choice of do you want to shoot with guns, or do you want to send drones after the enemy.
EDIT: Also, the civilian equipment that you get from tutorial is crap. Don’t use “civilian” stuff, get the good stuff from the market.