Wow, so many questions
It's a mix between a shooter and a RPG (you dont really choose your character, but it has elements of this genre). It's about a guy who wakes up and knows only one thing: He has to kill Strelok. That is for STALKER SoC. Good mods for Shadow of Chernobyl are Oblivion Lost and maybe the Arsenal mod if you cannot get enough weapons. Furthermore, Lost Alpha will be released somwhere 2013 I think. Difficult to say if it is a mod or a whole new game though. The second game, Clear Sky, is a pain to play if I am honest. The story could have been interesting but it's buggy, extremly annoying and frustrating at once. I only played it once and won due to a bug
The third one, Call of Pripyat, concentrates on a field agent trying to find out what happened to an airborne mission that went out of control. Misery is a really good mod for that one that will get a major update soon. If you just need visual enhancements, there exists a mod named Complete for all three games.
Tasks you have to do are killing things, finding artifacts, exploring forgotten secret labs (really scary if you play it for the first time, I can tell you that), many others and then there is the main quest, but I wont spoil it for you.
I'll give you some advice about the gameplay but not too much because learning is part of the game: Map a shortcut for medkits and bandages that you will find quickly (F1, F2; When I played it for the first time I always opened the inventory to bandage me and in that time I got hit another time so that I had to redo all of it
)
Second tip: Dont waste ammunition; especially from early to mid game there might be some ammo problems: Go for the headshot if possible (On my first run, I was always low on ammo, made the game much more interesting
). Thats all for now; maybe I will add some things later on.
EDIT: I knew I forgot something very important: Save often on new slots, the game does not do that for you; it will save you a lot of frustration, trust me