To use grenade launcher, first press the Grenade Launcher key, then mouse left-click (same button as when you'd shoot). Hit GL key again to switch back to normal shooting mode for your weapon.
Note: in multiplayer you can only use GL when you have it set as an attachment to your weapon.
Another useful tip. To use C4, first, press the C4 key, then right-click to throw C4, and left-click to detonate it.
To conclude, don't even bother using GL. Like nasabaer said, its for noobs, and on a lot of servers you will get kicked when you use it. Even when you won't get kicked, just don't use it. Same for martyrdom, last stand and LMGs. Don't use this perks/weapons.