Science & Industry

At the moment there isn't much variety in modules/rigs for citadels. Their fitting is quite straightforward:

High slots: Missile launchers and Energy neuts, for low/null you have some AoE weapons and for the XL citadels doomsday weapons.
Med slots: EWAR... Target painters, warp disruptors, stasis webs, dampeners, tracking disruptors. All up to your taste.
Low slots: Weapon upgrades, signal amplifiers, (currently rather useless) co-processors and reactor control units. Both co-processors and RCUs are not really necessary at the moment, since you don't utilize all of the citadel's CPU/PG afaik. Can't speak for the larger citadels and fittings for low/null, but in HS an Astrahus is fine without them.

Service slots: Refining array, clone bay and (for Fortizar and Keepstar only) Market hub. More to come in a later release, probably fall 2016...

Rigs: Bit of a variety here, you have refining rigs, weapon and e-war rigs. Depends on the citadel's purpose, those for Fortizar and Keepstar are combining bonuses of two medium rigs (Astrahus), e.g. refining bonuses for all minerals instead of only high/low sec ones.

And then there's fighters too. The Astrahus can only launch Light Fighters, Fortizar and Keepstar also Heavies.

That's basically it. Though I would wait a while before anchoring any citadel, especially in high sec... Too big of a target at the moment.

Fitting an Citadel is fairly simple, as there isn't a huge choice in modules, and things like PowerGrid and CPU are barely an consideration.

For fittings, you basically have two main option, make it effective against Sub Caps, or make it effective against Caps. This basically picks the highs for you, all you have to do is pick the Mids (all Ewar of some kind, I'd recommend Jammers in most cases).

For the lows you are probably always going to go with missile dmg/application rigs.

The main constraint in Citidals is Cap, you will run out.

One thing to remember, most people attacking an citadel are only going to attack it, if they have enough DPS and reps to survive your incoming damage. If you don't have an defence fleet of some kind, don't expect it to survive an attack.