xdEpicZombie, not that we want to scare you off about mods but if you don't know what you're doing listen, read and learn first.
1) There is nothing wrong at being a noob in the matter, we all where at some point, the main thing is that as a_slow_old_man mentioned you HAVE to read the licences, some games under certain
CC licences allow you to modify, re-use their materials under certain conditions.
If it is for a strict personal use you can do as you wish but be aware that if you intend to publish any non-authorised material on any respected AssaultCube site you will face the mighty power of the ban hammer, trust me there is no kidding here about this.
2) You can however if all licences permit it submit material found on the web but bear in mind that you won't get much recognition from it, we are a whole bunch whom have played video games, worked (freely or not) for them, cut a long story short.. have experience and we all know about these sites.
What is pretty much appreciated here is "Creativity", something that brings fresh air, new atmosphere to the game.