Kinda new player!

[Niwashi]

For Summoner the requirements are level 30 Arcanist and level 15 Thaumaturge.

Once you've met both of the level requirements and completed all the Arcanist class quests, there will be another quest at the Arcanist guild to unlock Summoner (as well as one for Scholar if you're also 15 or above CNJ). Job unlock quests (for ARR jobs) all begin at the guild hall of the job's main class, but then send you to an NPC somewhere else. In the case of SMN, it's in Old Gridania, near that guy who gives out achievement rewards. It's that second location that you'll then need to go back to for subsequent job quests.

And I about low lvl gear. What to do with it? Should I just sell everything?

You mean stuff you've worn in lower levels, but have since out-leveled it and replaced it with better stuff? If you've worn it in combat (or crafting/gathering), then it will be bound to you and the only place you can sell it is to NPCs who pay very little, so if there are other options, they're better.

If you used the item long enough for it's spiritbond level to reach 100% and the item is convertible (not all are), then the best thing to do is convert it into materia. As MistakeNot mentioned, there is a goblin, Mutamix, in Central Thanalan who can teach you to do this. Pick up the quest "Forging the Spirit" from Swynbroes. There's some RNG involved in which type of materia you'll wind up with, and some are better than others, but any materia is more valuable than used gear you've outgrown. In fact, if a convertible piece of gear isn't quite at 100% yet, but getting close when you get a better replacement, it may be worth continuing to use the old piece a little while just to finish off that spiritbond, so you can convert it.

If the item is a green or pink (aetherial) quality dungeon drop, you can also trade it in to your Grand Company for GC seals. You can do this regardless of whether it's bound. (I seem to recall there being an unlock for this, but can't remember quite what it was. One of the GC promotions maybe?)

If you're getting into the crafting part of the game, there's a corresponding skill, desynthesis, which also provides a use for old gear, letting you break down gear into other items (mostly crafting components). But you can only fully level up desynth skills for three of the eight crafting classes, so only those classes' gear types are useful for this purpose (and then only if its level corresponds with your current dysynth skill level).

If a quest rewards gear you don't need (which is common enough if you do quests below your level), so you end up with gear you don't have a use for, but have never used in combat, so it isn't bound to you, you can sell it on the market board to other players. That can nearly always fetch a much better price than the few gil that you'd get selling to NPCs.

If none of the above apply, and you're not going to want the gear again (for another class, or for glamour), then you're out of good options, so go ahead and sell it to an NPC. The few gil you get there is better than nothing.