Noob fitting guide...

Read the following if you really don't have any idea what to put your ship's slots.

Basic facts:
Amarr: energy weapons(lasers) + armor tank
Caldari: rockets and hybrid guns + shield tank
Minmatar: projectile weapons + shield tank
Gallente: hybrid weapons + armor tank

There are exceptions, some gallente ships use rockets but the lines above are rule of thumb.
If you want to be sure (and you want to be sure for sure since nobody wants to play a game wrong) you have to check the ship's details. Since it is written for noobs: open fitting menu and in the upper right corner you will see the ship's name. Next to the name there is a blue i. Click and read!

Let's see an example. Incursus... Description says:
"Gallente Frigate Skill Bonus: 10% bonus to Small Hybrid Turret falloff and 5% bonus to Small Hybrid Turret damage per skill level. "
Of course you can equip this ship with projectile weapons but in that case you will loose the bonuses. If your Gallente frigate skill is level 3 it means -30% damage. So a smart player puts hybrid turrets on. And you are a smart player, so you will put hybrids on.

Second round: what hybrid turrets?
The same turret for every place. It maybe sounds a smart move to put two blaster on and one railgun. In this case if you have to fight close, the two blasters kick ass and if you have to fight far, the railgun kicks ass. In fact there will be only one ass kicked and this one will be your. With this setup in close fight you will be able to deal only around 2/3 of your dps and roughly 1/3 from far.
The good way to choose your weapon: decide whether you want to be a close quarter combat guy or sniper. First case: put the same 3 blasters on the Incursus. Second case: put the same 3 railguns on.

Then comes the question: What happens if you decide sniper and your opponent comes close?
The answer: don't let it happen. Use the medium slots. Medium slots are for afterburners (AB), microwarpdrives (MWD), stasis webifiers and warp scramblers. The use of these equipment is obvious: makes you faster and your opponent slower. You will dictate range.

OK, now we dictate the range. It makes possible to maximize our damage. But what if he or she is a sniper as us? Then he will hit us hard.
Here comes the mitigation and repair part. The mystic tank word. Take a look at the list above. Or bellow:
Amarr: armor tank
Caldari: shield tank
Minmatar: shield tank
Gallente: armor tank
It means: gallente ships have armor tank bonuses only (or nothing but never shield), caldari are shield (or nothing but never armblablabla... So if you are a gallente and you plan to use gallente ships it is likely your ship will have armor bonuses. Therefore you will put armor modules(hardeners, plates and repairers) on your ships. And you will learn armor-tanking skills so you will put this kind of equipment on ships which don't have bonuses because you will be more effective with this tools.
Never, I repeat: never mix tanks on a ship. It does not make sense (not effective) to have shield and armor repairer/hardener/whatever on a same ship. It is not effective. Decide and stick to the plan. And always read ship info.

These are the basics. You can make a different choice if it makes sense. For example Dominix has the following ability:
"5 % bonus to Large Hybrid Turret damage per level."
but there are PvP fits without any turret, but neutralizers and vampires. The drones do the nasty part of the work.
Also a Dominix can be fitted to be a hull tank. Expensive, mad but working fit. And not basic.

Read this post and read your ships abilities. Try to fit your ship. Search loadouts for good fits. And keep in mind: if you cannot fit it maybe you need to learn support skills.