"CC" is an old school term meaning "crowd control," not a button press. Sorry for the confusion. I mean if someone tells you to "CC" the mob, they mean you need to use a skill that does Daze, Knockdown, or Stun. Which of your skills has these properties depends on your class and build. You will need to read your skills to know which do what. I'm not sure at what level you can access the Hongmoon Training Room (F12) which should teach you the basics of your class. You should have been taught some of this stuff by doing your Purple Quests with Hajoon back in the Viridian Coast.
When I was a noobie trying to learn my skills, it helped me to go to the Tome (K) and click "View Icon" down at the bottom. This lets you mouse over and read each of your skills based on key assignment. This can help you see which skills you need to be relying on. With that said, Assassin is one of the more complicated classes in terms of relying on combos and stuff, and I don't have much personal experience with it. Wish I could be more specific.
In terms of gear, your outfit does not affect your stats (though you do need to be wearing one), all your defense, crit, etc comes from your soul shields, weapon, and accessories. The storyline should give you everything you need until level cap. I think you should have the full Moonwater shields at this point if not the Silverfrost set.
Aaand it took me so long to type this, I see Deadly Intent has already explained some of this, but I'll just post it anyway. =P