when picking gear for your champion, the most important factor is the stats it gives, not getting the set bonuses. when gearing your champions, you should first figure out the main and substats you're looking for. if you have gear pieces that offer those stats and can get you a set bonus as well, there's no reason not to use them, but it's really not what you should be focused on.
the first thing you should do is make sure the gear you're using has the right main stats. for a dps champion (a champion whose primary role is to deal damage), you'll want crit rate or crit damage gauntlets, either atk%, def%, or hp% on their chestplate depending on which stat their damage scales off of, and speed boots. for most other champions, you'll want to use hp% or def% gauntlets, hp% or def% chestplate (or accuracy if the champion in question focuses on debuffing the enemy), and speed boots.
following the main stats, the next most important factor is to pick gear pieces with the proper substats. they follow a similar pattern as above. dps champions will want gear that boosts their crit rate, crit damage, speed, and their damaging stat. debuffers will want accuracy, speed, hp, and defense, and buff/heal focused champions will really only be concerned with hp, defense, and speed. of course, this is only a rough outline; there are plenty of champions that call for builds outside of those i mentioned above, but it should serve as a general guide for now.