I agree with above poster.
As you start farming continue keeping better and better gear and be more and more picky.
To begin with a chestpiece with flat hp migth be worth to keep if it roll alot of speed. But once you get further into the game this is what I would recommend keeping:
Gloves:
Primary stat should be: crit dmg/crit rate/def %/hp%
Notice that with crit dmg as primary stat it can be rather difficult to get a high crit rate. as such they are most useful for endgame gearing.
Very few champions benefit from att% if any on gloves. Mostly you would rather have attack champions have either surviability or crit dmg/crit rate in gloves.
Chest:
Primary stat: def%/hp% (att% for a rare builds like arena nuker or campaign farmer)
Boots:
Primary stat: speed (almost always. If you dont have speed as primary stats some end-game builds could have def%/hp% if speed is a substat).
Again a very few rare builds migth be the exception here.
I generel I think most find the consensus to be more or less the following priority of stats:
1. speed
2. Crit dmg
3. crit rate
4. def%
5. hp%
6. accuracy
7. resistance
8. att%
Do note however that the importance of stats vary from each champion. So you could use this guideline when deciding which gear to keep, but watch a video on your champion before equipping for a specific dungeon.
Also note that some stats complement eachother better. fore example crit dmg and crit rate complement eachother verywell. Often you would want a third skill being either def%/hp% or att% depending on the type of champion.
The same could probaly be said with def% and resistance. fore example if you build a tank with aoe provoke it might be handy to have def%/accuracy and resistance.
For many support champions that buff your own team all that matters is to buff often. So you want items with speed/def% and hp%. So you buff often and stay alive. The same most often goes if you have a champion with ressurection. It is vital that champion dont die first and often those ressurrect skills have a long cooldown. speed mitigates that somewhat.