I have an assassin at level 14 & I don't really understand what factions or guilds I'm allowed to join. I figured because I'm a thief/assassin it would only make sense for me to join the dark brotherhood and the thief's guild, which I've done. Would I be able to join the Stormcloaks and the College of Winterhold even though I'm not a warrior or a mage? Also, could any character join the dark brotherhood and succeed at it? Like a necromancer or a warrior in the dark brotherhood?

You can join any and all different guilds/factions and complete them at your leisure.

Any synonym of :

Archetype, playstyle, build, class, sub-class, no-class can join any and all guilds.

Probably be a little more rewarding to stick to the guilds that make the most sense for your character.

The Stormcloaks are actually part of the civil war, so you can *either* join the Stormcloaks or Imperials. As they're a military faction, they have use for all sorts, from diplomats to assassins, so you could join the Stormcloaks.

Remember- Stormcloaks have Skyrim's intrests at hear, while the Imperials have the Empire's intrests at heart,

The Companions are the closest to the Fighter's Guild that you can get in Skyrim, however.

You can join any of them and you can complete any of them. You don't really need to know any magic to complete the College of Winterhold. Maybe three basic level spells that anyone can cast and they provide them for you if you don't already know them. You can even complete the Theives Guild with a non-stealthy character. You can blast your way through many of the missions instead of sneaking. Of course, that also applies to the Brotherhood, though you'll probably have to pay a fine. And the Companions? They're just brute force, anyways. It doesn't matter how you complete their missions.

So, it's probably best to just choose the guilds that fit your character and stay with them.

My warrior is currently doing quests for Winterhold and the Mage's College. While he may become a spellsword type eventually, he doesn't currently use magic in combat, so it doesn't matter what your character's skills are, as all guilds will let your character join.