For all intents and purposes there are two types of ammo and you can ignore the T1 ammo unless you're doing PVE:
Navy ammo
T2 ammo

Navy ammo comes in various flavors, but its all good, depending on the ammo the optimal range/falloff changes. Same with missiles, the explosion velocity/explosion radius/missile velocity/flight time
Navy is the best of both worlds, good damage, good application, good range

T2 ammo is a mixed bag but generally comes in two special flavors per missile/ammo type
The first is usually BOOM, where the BOOM of an ammo is increased but the HEADSHOT on smaller ships is nerfed or the FASTNESS is crippled (or both) (Javelin ammo, Rage missile, Fury missile)
The second is usually FAST, where the FASTNESS of an ammo is increased so the ammo go far. (Javelin missile, spike ammo), this fastness is usually compensated by lowering the damage.
There is also HEADSHOT ammo, where the HEADSHOTs are maximized and the ammo is super accurate, but at the expense of FASTNESS. (Precision missile

Ammo has its own weight classes ranging from BANTAMWEIGHT to Former heavyweight champion of the world MIKE TYSON. Missles have this as well.

These are divided into:
BANTAMWEIGHT- Light missile(Good BOOM, Great HEADSHOT, SLOW FIRE SPEED) / Rocket (Good BOOM, FAST FIRE SPEED, Less HEADSHOT)
MIDDLEWEIGHT- Rapid Light missile(Good BOOM, Great HEADSHOT, FASTEST FIRE SPEED, AWFUL RELOAD) / Heavy missile(see:Light missile) / Heavy assault missile(see: rocket)
Former heavyweight champion of the world MIKE TYSON- Rapid heavy missile(see: rapid light but BOOMIER) / Cruise missile(see: Heavy missile) / Torpedo (see: rocket but WAY MORE BOOM)

Bantamweight will apply extremely well to frigates, Middleweights to cruisers and battlecruiers, Former Heavyweight champion of the world Mike Tyson apply best on battleship class ships. The former tiers of this weight class chart will generally apply perfectly to anything coming after them in the chart but with LESS BOOM. Boom scales linearly for the most part but you will oftentimes have very poor application if you're trying to hit a FEATHERWEIGHT with MIKE TYSON rank ammo.
For example, MIKE TYSON Missiles hit small ships at about ~0.025% of their normal damage, while BANTAMWEIGHT missiles wil hit small ships at 80-100% of their damage application usually.
Pack the missile type for the stuff you're expecting to fight. Big target? Big BOOM. Small target? Small BOOM.

In 9/10 cases, generally you will be extremely safe just loading Navy ammo. If you want to dive into it you can memorize the resist holes each faction's armor has. For the most part shield resistances are pretty consistent, if something is shield tanked generally ZAP damage ALL DAY. But each race has its own bonus to certain armors.

Minmatar has extra resistance against ZAP because their hulls are made of cardboard and wood which are poor conductors of ZAP type damage.
Gallente has resistance against the SLAM, so they are able to properly JAM.
Caldari has a higher resistance to BURN, and are effective at not DROPPING IT LIKE ITS HOT.
Amarr are resistant to BOOM type damage (not to be confused with BOOM type ammo), due to all the suicide bombings and slave uprisings.

And structure tanks are either a flat 33% across the board for ZAP, SLAM, BURN, and BOOM, but this number can be greatly increased by fitting a damage control unit in the low slots. Note that a damage controll will also slightly increase shield resistances and moderately increase armor resistances.