It's entirely possible i'm thinking too far past basic stuff, which i hardly consider anymore.
Sets is just a term for a grouping of skills; using a skill in one set gives a seven-second cooldown on skills of any other set, to discourage set-mixing. There's a handful of skills that don't belong to any set, but they have long cooldowns as a result. Any given move can be either internal or external damage, which matters for stats, defense, and various treasure/meridian boosts; it can be yin, soft, yang, hard, or taichi attribute, which affects if your inner gives it a damage boost; and it can scale by brawn, breath, dexterity, or a combination, which affects how much damage is added by stats. To say nothing, of course, of how any set gains max levels, although school fifth sets, faction sets, and subsect sets are fairly straightforward.
Schools have six (currently three on WT2) inners, each one stronger than the last by stats but not necessarily effect; ideally you want to level the highest one to max or close to it for raw stats. Each inner can also, though leveling (25/20/20/30/30/30) unlock a meirdian acupoint, allowing you to level the meridian higher; many people join and betray schools on a rotation to get as many acupoint unlocks as possible, but betrayal tours are another matter entirely. Schools have three basic sets, which they hand out like fucking candy, that vary from worthless to exceptional, that with effort other schools might get to a degree. There's also a fourth and fifth sets, unique to the schools and obtained by School Instances and Eastern Seas respectively, that're harder to obtain but stronger and usually better than the basic sets. Once you're in a school, the only way to leave is to betray, which rips out the school inners and sets; you can get refund pills when you join another school, which is how people can betrayal tour. You can also train away and disguse, but this only lets you join factions and other schools' subsects.
Subsects are quasi-schools: you're linked to the school (cant spy on them, can't script steal from them, can't war on them), but you can't actually use any school services. Subsects only have two inners, tier 2 and 4 respectively, and two sets, a basic that's cheaper and an advanced that needs a second trade-in item. You get level 1 of the first set and i think level 15 of the first inner free, the rest you have to work for. When you get a main school's meridian to 180 (currently not available on WT2) and level the subsect second inner, you unlock a reverse meridian of that school, which is like a normal meridian but stronger. You can leave a subsect by graduating, which takes a 100 certificate token, unless you get the second inner to 59 at which point it's free; when you graduate a subsect you get to walk with everything, even if you leave the main school one way or the other.
To summarize: school for merids and subsect for levels, unless you really like a school set or iner, in which case, you poor bastard. Consider specalizing in one weapon catagory and/or maybe one damage type for simplicity, but some combinations (internal quartestaff or external twin blade) only have about one cash set to work with (which aren't available on WT2).