Positioning is very important. If you're having trouble staying alive, try to play more defensively, at the back of the pack and observe the dynamics of the battlefield. Play a medium mech with a decent engine (80-90 km/h speed) and gravitate around assaults and heavies, but at a certain distance behind them, instead of alongside them.
Be patient, don't run off to the front of the pack as soon as the first target pops up on the minimap. Wait to get an idea of where the enemy is and for firing lines to form. Your only priority are lights that try to flank your assaults or heavies. Otherwise, wait for a target to present itself.
When moving, keep close to potential cover and move quickly from cover to cover to keep up with the team. Don't get caught in the open. Also, if you're moving down a natural corridor, don't walk straight down the middle, hug one of the walls on the right or the left. This limits the potential angles you can be shot at.
You mentioned you've got trouble hitting targets - try going for static targets at first. There's a lot of people that just stand around begging to get shot at. And always focus your fire on a certain chassis part. For starters, always aim for center mass, the central torso, forget the arms or legs as they're harder to hit. Aiming for center mass gives a lot more chances you will at least hit something, even if the mech is moving or torso twisting.
As others have suggested, lower your mouse sensitivity (I've got it set to .2, for example) and try to play the same mech for a longer time, you'll get accustomed to the speed and the weapons and will be able to better concentrate on the game. My advice about setups is going to go against the grain. Forget "meta" builds, you don't need them at Tier 5. Try to set up something basic that is easy to use - 1/2 ER Large Lasers or 1/2 AC5s (chain fire these and you'll hit stuff easier), supported by as many medium lasers as you can fit, but not more than 6; maybe an LRM or two. Group all weapons of a certain type together and you'll have no more than 3 groups, easy to use by binding them to the mouse buttons (left mouse, right mouse, middle mouse).
EDIT: I should place a disclaimer here that the "playing it safe" tactic won't work every time. There will still be steam rolls, either on your side or the enemy's, when playing it safe will have the net result of you still not doing any damage. But this is an approach to take at first, until you get a feel for the flow of different battles and where it is best to position yourself before the lasers start flying, so that you can later start playing more aggressively.
Edited by aaykeem, 07 January 2016 - 06:04 AM.