First of all let me get this out in the open, I cannot make you automatically better at the game. However, I will give you my advice and tips to improve you at the game.
Adjust to the size of the map whenever you're playing a large map don't expect one class to get you good scores, this means always have different classes for different sized maps for example if you play a small map use a SMG or even a Shotgun. With large maps LMG's and AR's are the best way to go.
Reflexes! How do you think John Marston was such a master with his reflexes? He grabbed the nearest computer and went to this website!
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My history teacher would be very disappointed if I said computers were around in 1911.
Learn the maps! Always go into a private game and look around the map itself to learn key escape points.
Keep your eye on the map! The mini map is a great way to learn the enemies position or camping points, don't be afraid to use a UAV!
Use weapons you're comfortable with! If you are more of a hit and run player I would use automatic weapons like the Vector or the MSMC. If you are a dead eye accurate player I would use high accuracy weapons that are semi automatic like the FAL or the SMR.
These are my main tips for now i'll edit this if I feel it needs more added to it.