Only one way to learn that.
Play the game and play different builds. It'll come naturally when you try to get stuff for yourself.
There is wide variety of builds and some may look for very specific stuff.
Some things you need to know thou:
Item can have 3 suffixes and 3 prefixes. These come on "tiers" - you can check them in game by holding alt when hovering over item in game or on some sites, i.e.:
http://poedb.tw/us/mod.phpSo, a good item is one that rolls with mods you want and those roll on high tiers.
For example for strength based boots, resistances are suffix and tier 1 is 46 - 48%. Life (90-96), movement speed (30%) and hybrid armour life are prefixes.
So 25%+ movement speed boots with over 100 life with 3 high resistances mods (35%+) is a decent pair.
In general weapon, amulet and quiver are the place to get offensive stats.
Boots, helm, armour are more for defensive stats, although it depends on build.
Rings, belt and gloves are mix of defensive and offensive depending on what you need.
Effective life is good on all slots - so life / es / (possibly mana - again depends on build).
Resistances you can fill out on i.e. jewels so you have max resistances for different map mods.