*hands out his complimentary LRM hat* Welcome to the forums!
I've tried to get most of this info in one post (albeit, it's a bit large and some of it isn't accurate anymore as it's changed along the BETA progression), but I would suggest suspending your previous knowledge of MW games and approaching this as a reboot and not just Mechwarrior 5.
Heat, Mech control, getting rid of Alpha-ing Disease, can be difficult at first, but after awhile it's actually not bad (I do like the torso twist and actually having heat management be a part of the game).
Yes it's still BETA, so we are working out a few (or a lot depending on who you talk to) bugs and still tweaking things (like the brutal new player punitive economy).
Trial mechs are a bit underpowered, so understand they are really your 'training ground' you are not expected to post a +5 K/D/R in them (surviving is the first step).
Many, many features are in the pipeline, that should add a ton of depth to this game, the community is active (sometimes a bit too much so) and while you will get some heat for posts like this, generally we're pretty helpful.
Ask questions in game (it's best to start out with 'alittle help plz, I am new') it helps set expectations for the match.
Also you'll run into a few groups of players, we call them evil Pre-mades

, they usually do quite well, I suggest getting to know some of them and getting voice com (C3) installed and running with them once you're up to speed (it's a different game experience when in a group working together).
Good Luck, and Better Hula!