Seriously, the advice about not spending those new-to-the-game CBills is the best advice you'll get. I spent a fair chunk of mine figuring out how the customization system works doing things like buying an engine only to realize how hellishly long it takes to earn the couple million spent on it. That was my first big newcomer mistake.
Additionally, I too was INCREDIBLY FRUSTRATED knowing there is a perk in the game, that makes your enemy be able to hold target on you for 2.5 seconds even after you're gone from vision, which is prohibitively expensive to a newcomer... I thirsted for a radar dep module for many a salty tier5 game. In fact LRM's are still annoying, but not as bad as AC boats aiming for the head making my screen black with smoke and shaking my mech to the point of blindness. (and I have to replace radar dep with gyros to stop the shake? eff that crap....)
Second big mistake I made starting out was buying a King Crab(100 ton assault IS mech) thinking I'd be cool, tough, and all-powerful only to learn quickly how easy it is for a light such as a Jenner or Arctic Cheetah to run around your back and small laser you to death without you being able to do ****. This was compounded by the fact that most people didn't look out for teammates AT ALL in T5, and can't hit lights because they're not used to aiming in the game.
I probably drifted off topic a bit...
Dealing with LRMs is still frustrating in the top of T4 (lights spotting you out for their team) and you deal with this in the aforementioned ways:
GET COVER-Find a building(like on Crimson Strait, or Grim Plexus) and put it between you and where the missiles are coming from as soon as you hear that awful "Incoming Missile" message. On other maps like Canyon Network, the natural landscape has nice plateaus that work great for this. Some maps even have nice places with overhead coverage to sit under while your team figures out where to move from.
MAKE FRIENDS WITH ECM BUDDIES- If there's an ECM mech on your team, be his friend. Stay close by(within 90m) but don't sit behind him all the time (people often like to peek&poke, then reverse back behind cover, don't be trying to poke them from behind, it's not polite). Defend your teammates, and move together.
ALWAYS BE ON THE MOVE- if you hear that "Incoming Missile" alert, you better be moving to cover. Assess where the missiles are coming from and use the above tip. If there is no cover attainable before you really start getting pummeled, run to your team, and run like you're running from bees with left and right jukes. One teamie might have AMS to help, and one might have ECM as mentioned above. Once you have Radar Dep, if you hear that damned alert, start running out of vision. They will lose lock quickly, and the missiles will land at the last place the enemy had lock of you.
KILL THAT PESKY LITTLE SPOTTER- This is a considerably harder to do while your screen goes thick with smoke and your mech shakes uncontrollably. However, if you can take down that one little mech, you will be saving your team considerable effort. If you can't kill it, get the heck out of its vision/lock so their team can no longer rain on you.
I started playing this game towards the end of last year, and I'm by no accounts a veteran of the game. These techniques though are what really save my *** from the LRMs so long as I don't let the "Stupid Target Decay module *grumble grumble*" "F2P but Pay-to-Win model" or "WHAT THE **** WHERE IS THEIR SPOTTER!?" thoughts get to me.
Enough rambling on.
If you REALLY feel like you can't take it, join em.... play the stalker missile boat trial mech thats up, and be the lame-o that cause so many people trouble. Sometimes you can't figure out counterplay until you play the thing you don't think you can counter.
Read the forums, use resources like
http://metamechs.com/ , and consider chatting with players over voice chat about tips tactics and whatnot.
Enjoy, and happy giant robot killing!