Shields vs Armor vs Structure
* Armor has the best resists, but is still weak against explosive and kinetic damage, shields are weak against Electromagnetic and Thermal damage, Structure is weak against everything.
* Shields regenerate slowly over time and automaticly repair at stations, armor and structure only get repaired if you use the station's repair services (if the station has such services, and they're not usually free).
* Shield modules use mid slots, armor and structure modules use low slots.
* Shield modules sometimes make you a bigger target and easier to hit, armour and structure modules sometimes slow you down and make you easier to hit.
* When structure takes damage, your modules might also get damaged, using structure as your primary defence is not advised.
The general rule is that Caldari ships are all about shields, Amarr and Gallente use armor, and Minmatar ships vary depending on the ship type.
For example the Rupture has 5 low slots and 3 mid slots and slightly more armor, so it's generally going to be toughest if your fit it for armor. However the Thrasher has more mids and more armor so it's better off with a shield tank.
The Stabber is one of the few ships in the game that gets a speed bonus, it's clearly designed for speed, so any armour plates you put on it will slow it down, so it's best to try and buff its shields using its midslots....
Those mid and low slots aren't solely the domain of things that make your ship tough, most ships will dedicate at least one mid-slot to an afterburner or microwarp drive, and those low slots are where your damage boosting modules go. Electronic warfare goes in the mid-slots including the all important Warp Disruptor that's essential to stop your target from running away. As a result some ships may find themselves tanked differently depending on their role - my favorite example being the Rifter.
For PVP a rifter is generally finds its midslots filled with a microwarp drive, warp distruptor and stasis web so the tank goes in the low slots and buffs the armour. However for PVE the web and disruptor is unnecessary, which means you can fit a shield booster and hardener and leaving the lows for gyrostabilizers to increase your damage and nanofiber structures to increase your speed and agility. Either way the rifter's best defence is actually its speed and ability to get in under the guns of bigger ships and so regardless of fit you need to be bold and fly this fast and close to your targets.
Speed tank is basicly reducing incoming damage by avoiding enemy fire, you can evade turret weapons by moving perpendicular to the target, that's what the orbit button does. It's the angular speed that matters, so travelling at 1000m/s in a 10km orbit is equivalent to travelling at 500m/s in a 5km orbit or 2000m/s in a 20km orbit. The bigger the guns the slower the tracking, so just like the X-Wings attacking the death star could avoid the primary defences your little frigates can run circles around battleships with relative immunity. For missile weapons the damage formula is slightly different, but generaly, speed keeps you alive.
For new pilots with relatively few skills the speed tank is universal, set your ship up with an afterburner (one of the first skills a noob should train) and short range weapons. Orbit your targets as closely as you can and they'll have a hard time hitting you, it's easy and effective, but many new pilots prefer to fight at range due to unfamiliarity with the mechanics. I'll tell you though that my 5 year old daughter has killed hundreds of npc targets this way without losing a ship yet, so have faith in the speed of our small ships and you will prosper.