I am playing COD 4 : modern warfare now for about a month, I still have the very basic weapons and I see many players with advanced and customized weapons ( I can pick them when they are dead and use them and they are way better than the ones I use ).
I want to know how to unlock/open/get such weapons ?
Weapons are unlocked by playing the game and ranking up. Attachments and other accessories are earned by challenges with said weapon.
Source: Experience & this link.
When playing online, you'll gain XP after completing games. You'll also get XP for doing challenges, killing enemies, planting bombs etc...
Once you reach certain levels, you'll be able to customize your loadout with unlocked equipment.
Before entering the game there is "create class" button. There I can create my own custom class and use all the weapon I've unlocked. And then when logging to a server these newly created classes appear and can be used to play.
That's what I was looking for.