"The only true wisdom consists in knowing that you know nothing."
- Bill S. Preston, Esq.
It's easy to get turned around, be sure to keep an eye on your team so you don't get left behind. If you want to help out your Team Lead, stay in sight and stick together.
The best way to stay alive is to stay away.
When moving: Try to keep at least 10 meters of space between you and your teammates.
In Cover: Do your best to spread out, but prioritize cover over spacing.
While you should be making sure you're not falling behind, you should also be making sure no one else falls behind.
During a mission, you'll likely be assigned a color along with someone else. This is your Battle Buddy, keep an eye out for them and they'll keep an eye out for you.
You're Responsible For:
Backblast is a cone shaped area behind a rocket launcher where hot gases are expelled when the rocket is fired. While this isn't modeled in Vanilla gameplay, it is with mods.

Everyone around them will make sure that they're not behind the launcher and that there's nothing behind the person firing the rocket.
Once the people around them have confirmed it's clear, they'll reply "Backblast clear!"
The launcher is now free to fire
Overpressure is almost exactly like backblast... but in reverse. Overpressure is a cone shaped area where hot gases are expelled when a large round (often from a tank) is fired.

Even if it's your first Op, or you're unsure, don't be afraid to call out targets. It's not friendly fire until someone pulls the trigger.
Anything you think your team isn't aware of!
Information is key to mission success.
Remember the Three D's! Diners, Drive-Ins and Dives
Tell us what you see!
This can be anything you think your team isn't aware of!
Call out the direction you see contacts! Ideally you'll give the Degrees on a Compass, but Cardinal Directions (North, South, East, West) are just as good when you're moving or it's urgent.
Finding Degrees:

In the above example, the contact is at 190 degrees
Try to estimate the contacts distance in meters. If you don't feel comforatble giving a number, just say Close, Near, or Far.
This is just a guess, don't worry about being too accurate. The imporant part is letting your team know you see something!
Smoke Grenades are used in conjunction with radio calls to mark friendly or enemy targets. While this can change due to mission circumstance, we typically use Red, Green, and Purple smoke grenades to mark points of interest.