I've just just a few critical points to point out here, in addition to some changes from what Azaual has written. While this is a GREAT guide, there are some things that should be changed around.
First off, the most important points:
1) NEVER FLY SOMETHING YOU CANT AFFORD TO LOSE!
2) NEVER MIX TANKS.... EVER!!!!!
3) NEVER MIX GUNS... EVER!!! (exception: launchers can go with turrets... but not 2 different turret types)
4) YOU CAN NEVER HAVE ENOUGH CAP!!!
I'll explain further the different points...
1) If you have a faction ship setup with faction modules (or even just T2 ship with T2 fittings), and you lose it, thats a major hit to your wallet. If you aren't able to replace the ship easily, its too risky to fly. This rule will keep your wallet happy, as well as you happy when you do go boom!
2) NEVER mix tanks... if you are Caldari you tank shields. If you are Gallente, you tank armor, and so on with the other races. Look at your ship's bonuses to see what tank they use. If your ship gets a bonus to armor repairers, then tank ARMOR. If you get a bonus to shields, tank shields. If you dont get any bonus, but your ship generally has more medium slots, then tank shields (unless you are in a specific race where that entire race only tanks armor/shields). If your ship has more lows, tank shields, but that depends on the race as well.
Here, Azual has some misleading information.
Active Armor Tank - An active armor tank consists of 1 (or 2) armor repairers, as well as Armor Hardeners. The damage control is not considered an active tank, nor a passive tank, meaning that it'll work in either case. Hardeners use cap to run, but give a much higher resist bonus than other modules such as plating, etc... The definition of an active tank is that it uses CAP. You'll want a lot of Cap Rechargers in the medium slots, and possibly even a Cap Injector to give even more cap. DO NOT PUT CAP CHARGING MODULES IN THE LOWS!!! USE THE MEDIUM SLOTS FOR CAP RECHARGERS!!!
Active Shield Tank - An active shield tank consists of the same thing as an active armor tank, except you'll be using a Shield Booster and Shield Hardeners. The only problem with active shield tanks is that they use a lot of cap as well. But, since shield tanks use medium slots, you cant fit Cap Rechargers. And the low slot alternative for cap regeneration has a side effect of making your shield boosting worse. In this event, you'll NEED to have a Cap Injector in one of the medium slots, in order to provide as much cap as possible, so that your tank will last a while.
Passive Armor Tank - AHAHAHAHAHA!!!! There isnt one... well... persay... the ONLY passive armor tank that can work would be to use modules such as a damage control, as well as adaptive nano membranes, etc... AND using plates. This is the ONLY time you'll want to use plates. The plates slow you down and are generally bad, since it would be better to have more resists, than more health. However, for this to work, you'll have to use a technique known as "Spider Tanking". This means that (assuming you have 5 people in your gang, all passive armor tanking with a remote armor rep in their high slots) when you start taking damage, your allies all target you and start using their remote reps on your armor. As different people are shot, the group targets the people taking damage and all use their remote reps on that person. This means that you are never healing yourself, but relying on your team to keep you alive... and your team is relying on you to keep them alive. This is an advanced PvP tactic that needs to be worked on extensively if you are planning on using it. This only has an advantage over active tanks in that if you have neuts or vamps/nos on you (the ones that either drain the enemies cap, or gives your the enemies cap), and you lose your cap, you can still fight and get healed by your team members. THIS FIT IS UNEFFECTIVE WITH LESS THAN 3 PEOPLE IN THE GROUP, AND MOST CERTAINLY THE WRONG TANK TO USE IF YOU ARE BY YOURSELF!
Passive Shield Tank - The whole point of a PASSIVE tank is to use NO CAP AT ALL! By fitting a hardener at all would be mixing tanks, WHICH IS A BIG NO NO! Azual is wrong in that point, but the rest of what he says is correct, with exception to the Shield Extenders. For a passive shield tank, you want your shield recharge rate to be as fast as possible. In theory, the shield recharge rate is so fast that its almost like having a permanent shield booster on your ship. Here, your goal is to reduce the recharge time as much as possible, as well as trying to keep as much resists as you can get.
Again, the point to a passive tank is to use no cap, so that if you are drained, you wont have to worry about your tank. Also, if you setup your ship right, you can use a passive tank allong with neuts and vamps/nos in your high slots, instead of a full gun rack. This will allow you to drain all of the enemies cap, and crush their tank, while you will still be happily tanked passively.
3) NEVER mix guns... Azual says this by saying "all your weapons should have the same or similar ideal range", but doesnt explicitly say it. When i say dont mix guns, im not saying that you can only have this particular named railgun, but ANY railgun within the same class (i.e. 75mm OR 150mm) Do not put a 150mm with a 75mm. If you cant fit another gun of the similar type, fit a remote armor rep, remote shield booster, neut, vamp/nos, salvager (not in PvP), missile, etc... if you cant fit ANYTHING else, then you can put on a smaller gun... but that should be your LAST resort.
4) It is what it says: you can not ever have enough cap. If you have any vacant slots, with an active tank, try to fit something that'll give you extra cap, such as a Cap Recharger in the middle slots for an active armor tank. For active shield tanks, you gotta be careful what you put in your low slots, since some of the cap recharge rate modules have a side effect of hurting your shield boost rate, WHICH IS BAD! So, check to see what you have setup, and make sure it doesnt hurt your tank.
4a) If you are going for PvP, consider modules such as sensor boosters, webs, warp disruptors, etc... Since those will be more helpful than cap chargers, most of the time. NOTE that most EW is pratically ineffective against NPC rats! Webs work, but warp scramblers are pointless, since NPC rats dont ever warp away.
If you have a question about a particular fit, post it and ask to see if anything can be improved. After time, you'll get much better at putting together good fits for different ships. I've developed a feel for how to fit different ships, but its taken me some time. Point is, it'll come to anyone, as long as you have someone to show you several good examples of fits that will work.
NOTE: THE SHIP FITTING WEBSITES SHOULD NOT BE TAKEN AS BEING GODLY FITS!!! THERE ARE STUPID PEOPLE THAT PUT UP THEIR JUNKY, MESSED UP FITS ON THESE SITES!! USE YOUR JUDGEMENT, AS WELL AS SOMEONE WHO KNOWS WHAT THEY ARE DOING WHEN TRYING DIFFERENT FITS!!!