For my mod I want to make a item that on "f" key pressed will despawn the item and give the player a different item. How is this done?

You can create a keyhandler.

You can use ClientRegistry#registerKeyBinding to bind the keys you want.

 

Forge Wiki Page:

http://www.minecraftforge.net/wiki/Key_Binding

 

PM me if you want further help, or just reply to this post for others to help you.

Good luck c:

"You can create a keyhandler."

And that is only the beginning...

 

You will need your KeyBinding to sent Packet to server (Bindings are client-side). The server will have to operate with data sent, which is sender.

You will have to pull inventory from sender (player) and set current slot to different item (current = in hand).

player.inventory.currentItem;

Remember to 1st check if slot is not null. To override slot simply use something like setInventorySlotContents() (Idk if it's exact name)

 

Packet tutorials:

http://www.minecraftforum.net/forums/mapping-and-modding/mapping-and-modding-tutorials/2137055-1-7-2-customizing-packet-handling-with

Shorter:

http://www.minecraftforge.net/forum/index.php/topic,20135.0.html

Where do u put what the key does?

public class KeyHandler
{
public static KeyBinding pawnItem = new KeyBinding("Pawn Item", Keyboard.KEY_K, "key.categories.hud");

public KeyHandler()
{
	ClientRegistry.registerKeyBinding(awnItem);
}

@SideOnly(Side.CLIENT)
@SubscribeEvent
public void playerTick(PlayerTickEvent event)
{
	if (event.side == Side.SERVER) return;
	if (event.phase == Phase.START)
	{
		Minecraft mc = Minecraft.getMinecraft();
		if (pawnItem.getIsKeyPressed())
		{
			//send packet to server, on receiver side (server) you can get sender (player) and modify his inventory
		}
	}
}
}

I am still really lost I have no idea where to put what the key does.

Do I put in in the class of the item?

I am still really lost I have no idea where to put what the key does.

Do I put in in the class of the item?

 

No.  You create a NEW class that is your key event handler.  You then have to do all the networking stuff, which is likely going to involve (at a minimum) two more classes, IIRC: the packet itself and the packet handler.