HUD Breakdown:
Top Centre: Time left in game, with how many mechs killed on each team. The Yellow bar represents how much of your point has been capped (friendly), red is the enemy capture point.
Top Right: The current mech you have targeted and gathered info on. His current armor status is displayed (both front and rear) and it will show all the weapons he has equipped. Weapons displayed in red mean they have either run out of ammo or been destroyed.
Centre: + reticle is your torso aim, O reticle is your arms. The number to the right of this is a range finder showing the distance to what you are currently targeting.
Lower Left: You current mechs armor status. The inner shaded area is the internals, the outline is the armor. Transitions from Yellow --> Orange --> Red ---> Nothing (once destroyed).
Bottom Centre Left: Your current speed. Meter goes up when you throttle up, the blue represents where you have your speed throttled to (mechs take time to get up to speed), orange is where your speed currently is.
Bottom Centre: Battlemap. Friendly mechs in blue, enemy mechs in red (but only show up if a mech on your team has Line Of Sight on them, or they have a NARC or TAG on them). The viewing triangle shows your Field Of View relative to your mechs position.
Bottom Centre Right: Current heat. Run hot, but don't let it hit 100% or you will shut down.
Lower Right: Your weapons groups. Mouse 1 Control group 1, Mouse 2 controls Group 2, 1-6 on the keyboard control their respective groups. Typically, put energy weapons in one group, and seperate LRMs, Ballistics, and SRMs. Make as many groups as you need to manage heat. Move the selected group/weapon with the Arrow Keys, and Assign/Unassign weapons with the Ctrl key.