Set graphics to lowest, if you can load the game faster you have a higher chance of choosing your desired class when a match is about to start.

Read some guides. I have posted several screeds with tips to play if you search. You do not need a sniper rifle. The objective is key, not getting a few long-distance kills. The game doesn't even show your deaths on the scoreboard to encourage players not to think in team death match K/D terms. Look at the points, and you'll see how much more you score for playing the objective than simply getting kills.

Plenty of new players jump into picking the sniper because they have insecurity issues and want a power trip. You will very rarely see a sniper with a high score, unless they have a bayonet fixed and have a lot of pistol time because they're in the zone. Most often the round finishes with the sniper at the bottom with a score not even in triple figures. After grenades, sniper TKs are the most common, because of idiots with a giant telescope on their rifle being unable to distinguish uniforms.

Play rifleman, get better. Add in the other classes as you get to understand the game's flow. That's really all there is to it. Follow the SL and always try to get into the objective.