I've been playing the BF3: Beta since the launch, but am really struggling.
My K

ratio is 0.5. I am want to know what the best game is that I can play to practise and increase my skills in this genre? (I'm on PC)
Best advice I can give you here, is to take it slow, and stick to one game.
The vast difference between two games that might seem similar on the surface, can cause things you learn in one game to not apply to the one you really wanted to play.
A few things that a lot of new FPS players stumble over, I will lightly touch here for you.
Objective Tunneling.
OT is the process whereby a new player would make a mad dash for a certain target, goal, piece of cover, whatever. The mistake they make is that their observation becomes limited only to the object in the center of their mind, ignoring everything else.
Always survey your surroundings. Constantly. Look left. Look right. Look left again. Look up. Look right again. Look down. Look up again. You should at all times keep a constant vigil on your immediate and semi-long surroundings.
Know where your teammates are. Know where your enemy might be.
Yes, you must reach your objective. But reaching it dead serves no purpose whatsoever. Take your time at first. Crouch, crawl, whatever.
SFiA/R (Shots Fired in Anger/Reflex)
Be very careful of this. A lot of people react to an enemy in an FPS by simply holding down the fire button. You need to overcome this urge/reaction. A good way to counter it, is to practice close-up kills with pistols or shotguns. Your mind will become more at ease with things jumping out at close range, and you will be able to relax a bit better on the trigger-finger.
Controlled bursts will always win over spray-and-pray.
Bee-lining.
Never. Ever. For whatever reason... run in a straight line.
Weave. Duck. Dodge. Slow down. Crawl for 2 seconds for no damn reason. Jump left just coz you felt like it. Don't let your movement quirks become predictable either.
The opposite center of this is when new players suddenly stand still when they need to engage someone in combat. While, yes, you do need some amount of stand-still for accuracy, few players realize that your accuracy will be fine if you simply crouch while moving and shoot.
The alternative is burst-move-burst-move-burst-move-reload-move-repeat.
Familiarity with maps and the hiding spots etc comes with time. Don't worry too much about that. Some people learn a map instantly. Others take weeks or months.
Lastly. As a new player, I highly suggest playing sniper 30% of the time. Except here is the twist. Don't shoot. Just watch. Mark targets while you're back there. Spot things for your team. Report movements to your squad. Learn by watching what others do.