If you're a half (or quarter!) decent FPS player, I recommend persevering. I'm sure I remember most of us at the start of this thread saying that we sucked and weren't getting kills. But when you start to get an idea of the various weapons (and which ones you like), and by extension what ammo you need to be looking for etc., I imagine it'll start to come together for you. A breakthrough for me was understanding the attachment interface too, so when you're in front of an item on the floor I knew if it was applicable to one of my weapons. Probably massively obvious to other folk, but I know that I didn't understand it all right away.
You're right in that there is a lot of game-specific knowledge you need to develop, but that just comes from getting involved, running about the maps, learning where clusters of supply bins are, getting to know the characters abilities to suit your own style /preferences and whatnot.
Getting stuck in like that is just more enjoyable once you've got a couple of weapons you know you can use well.
