It depends where you are at in your modding, but some simple things to consider.
Speed is king - always. It determines how often your characters take turns - more turns, more wins. Pretty well all the time (there's a few exceptions... very very few)
Simple principles:
1) Level up mods to see all 4 secondary stats
2) Dispose of any without a speed secondary stat (except arrow - keep speed arrows)
3) Depending on depth of your mods you can also trash those with only +3 speed or +4 speed - if you're just starting those are better then one's with no speed secondary.
4) Level up to gold - if they roll speed more then 1X then they're likely keepers - lock them so you don't by mistake sell them.
5) For potency guilds, you either want potency primary (cross) or potency secondary stat. Similar for tenacity mods with obvious changes.
6) Figure out and save mod filters - new mods (< Level 15), mods to upgrade (grey, green, blue, purple 5-star with speed) - start upgrading those. This will just make finding mods that you need to upgrade easier. Ask if you need help with some useful filters.
7) Watch the video recommended by DaarthBrewer - watch the whole series - take notes. Then watch it again. It's still highly relevant.
8) As you get better mods, upgrade your top characters with those - put the +25 speed mods on your GLs or key characters... then take their hopefully good mods and put them on your B-characters, etc. It's trickle down economics...
Improving mods is a LONG game - marathon not sprint. You should always be working on mods. If your in a guild with Discord and Bots the OmegaBot has a useful /modscore command.
That's a quick-ish summary but feel free to ask questions.
