Before you try to battle people in multiplayer, you need to know the territory. To go explore a map without anyone joining, create the game in LAN (Local Area Network). This keeps the game private to your house.
Thoroughly explore each map, making notes on what weapons are where, and where the power ups are. Also note where the best place for cover is.
After you know the territory like the back of your hand, you need to find a good weapon combo. Proceed to step 3 to find your perfect combo