Welcome!
To add onto MoonUnitBeta's comments:
-ECM basically hide friendly mechs from radar detection when spotted by the enemy. This means you can not lock on to the target nor see their info when they are still with in ECM bubble. If a mech with ECM comes close to you, you lose most targeting information other than the location of your friendlies. You will also not be able to use LRMs when you have an enemy ECM mech near you. You should definitely brush up on ECM before you go into battle. I personally seen the implementation and tweeking of ECM since its introduction and find that is may be difficult for newcomers to get use to.
-ERPPC and PPC can disable ECM for a few seconds.
-Gauss Rifles are extremely fragile now and can explode when damaged.
-Artemis, TAG, and Narc components help guide your LRMs better for maximum damage.
-JumpJets shoot you up faster allowing you to get some quickshots off.
-Some mechs now have missile bay doors that you need to toggle "/" to open and close them. There are some indicators in your cockpit to show this. With doors close, your missiles will come out after a brief delay. The doors will prevent some damage to the weapon system when close.
-The modifiers that you can unlock with your EXP in the Pilot Lab actually work and are very useful. However, you will need to unlock the "Basic" modifiers for 3 different variants in order to advance to the next tree.
-Modules and Consumable are available to unlock and purchase for your mechs. These are like the modifiers I mention, but you will need to unlock them with GXP first and then purchase for your mechs with c-bills. Mechs also only have so many slots to fill with modules. Modules will enhance your sensors usually. Consumables are available in the form of coolant flush and some fire support. However, fire support consumables are not worth your c-bills or MC right now. Avoid the following modules and consumables for now:
Advance Zoom, Arty Strike, Air Strike, UAV Drone
-Machine Guns, LBX-10, and Flamers have an additive critical damage modifier that can deal massive damage and destroy your weapons and heat sinks once your armor is gone.
-There is a new game mode called Conquest where you can secure points to win.
-There is also a Training Grounds game option in the "Launch" menu that also you to explore various maps and test out your mech alone.
-You can enable arm lock in your options menu. This helps make your shots from your arm and torso converge better.
-Whether you choose to grind c-bills with your founders mech or trial mech, or purchase a Hero mech, be sure to do your research and the mechs and utilize all tools available to you.
http://mwo.smurfy-net.de
The above link is a very useful database that you can use the web base mechlab to design your mech before you buy it. You can also share the mech design and ask for opinions.
Now that will be the end of my contribution.
Good hunting Mechwarrior!
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for all your matches you use it in! 
right away, play around and get a feel for what you like. Hero Mechs are pretty cool but you should make sure you feel you would enjoy that mech's speed, loadout, and overall style. different Hero Mechs are goood for different things.
