How modules work in this game:
Ships are vehicles that allow you to undock and get into space. Each ship is designed for a role (DPS, Tank, tackle, electronic jamming, remote repair (heals), scouting, industrial transport, mining, etc.), so when you get into a ship, you basically assume that role (and look for PVP or PVE encounters where that role is a success). Because your character can unlock many ships, you’re basically unlocking the equivalent of a whole account of characters (fighter, mage, healer, etc.), all the roles you want you can unlock on one character, or create alts if you want.
In any case, the ships are basic vehicles but they don’t have any “abilities” until you install modules to them. Modules have abilities: weapons shoot or otherwise do damage, shields / armor can repair damage to your ship, afterburners or microwarpdrives make you go faster, various electronic jamming modules let you jam things, etc. You unlock modules like you unlock ships, by training the various prerequisite skills. Modules that you install in your ship will appear as buttons that you can activate once you’re in space.
Ships have the following stats:
various slots for installing modules - high power slots, med power slots, low power slots. You’re limited by the number of slots, and there is nothing you can do to add more slots to a ship (you can switch to a different ship that has more slots).
power grid and CPU - modules also require power and CPU cycles to be installed into the ship. The power and CPU are limits - you won’t have enough power to install cruiser or battleship-sized weapons, shields, or armor, to a small frigate ship, even if you have the slots. So when you’re looking at modules, keep “small” modules for frigates and destroyers, “medium” for cruisers, battlecruisers, industrials, mining barges, etc., “large” modules for battleships, and “XL or capital” modules for capital ships. There are only two skills to increase the Power Grid and CPU on all your ships: Power Grid Management, and CPU Management. Train them to max. There are several modules (reactor controls, micro auxiliary power cores, co-processors) and rigs that can increase the ship’s power and CPU, but they use up slots, so it’s a trade-off.
capacitor energy (juice) - some modules require this energy to fire or function; the capacitor recharges itself and the modules and guns suck this energy from it. It’s similar to mana usage for spellcasters. There are a few skills in the Engineering category, and several modules that can increase the capacitor energy that’s available, and there are a number of skills that reduce the consumption of your guns, defenses, and other modules.
ship bonuses / ship description / ship traits. Most ships will have an indication to what they’re designed for, if you read the descriptions. For example “20% bonus to damage if you use Small (Amarr) Laser Weapons” is a significant DPS bonus for using those weapons, as opposed to Minmatar Projectile guns, on that particular ship. You, of course, have freedom of choice (if you can fit within the ship’s slots and power grid/cpu), and sometimes people fit differently just to surprise their enemies, but in the majority of the cases, 20% extra DPS is much better than “surprise!”
The game has several sources of information:
There is a Ship Chart applet somewhere on the tool bar on the left side of the screen, where you can right-click any ship and see its stats and info.
The Market, which you can open from anywhere, lets you use a search function or browse any object in the game, and you don’t have to buy; you can just right-click and “show info” to see its stats, prerequisite skills, “what it’s used for”, etc.
The in-game Fitting window has a “Ghost Fitting” button where you can mock / pretend to fit any ship with any modules, and see its performance stats (DPS, tank, speed, etc.) as fitted.
For specific guidelines for how to fit a ship, I would recommend EVE University’s guide. EVE University is a player corp., and they have a very good wiki, and also classes and video tutorials if you want more info. Feel free to ask questions here, too, and we’ll try to answer.