Regroup with Rhino Squad
You'll start on planet's surface, near the crushed landing module (#1). The soldier walking near your position will acquaint you with first in-game order - you must find all your teammates. Go in big building's direction. You'll meet medic Anderson (#2). Wait until he heals you and go inside the complex.
Few seconds later you'll find yourself in the room and encounter first enemy (#3). Help allied troops in exterminating some opponents and go straight ahead, by taking the only available path. During the walk, look in crates (#4). You can find there health and ammunition.
In the next room, you can pick up a machinegun (#5). Use it to clear the path of enemies and go straight ahead. Check the doors (green lights on them mean that you can go inside). Sometimes doors will lead to small rooms, where you should find health and ammunition.
After reaching the team (#6), you'll see a wounded soldier. Allied unit's captain will ask you to get Anderson (medic from the beginning of the game). Head back the same way you came, but be aware of running into more enemies. One of the Stroggos will hit marine with a blade. Kill the Strogg and take the opened passage. You will meet Anderson there (#7). Help him with fighting and (together) return to a wounded soldier.
Try to remember that Anderson can heal you, if necessary. To do that, click on him, when the surroundings is clear of enemies. After going into next room, you'll encounter new enemy - kill him immediately. Get close to the place from picture (#8). Below you should see a small bridge. Armor and some ammunition await you there. Flip the switch (#9) located near the ladder (it'll open the red gate) and climb back up.
Move straight ahead and eliminate next opponents. After reaching the team, Anderson will heal a wounded soldier. Wait until one of the allied troops will open the door and walk outside the complex. You'll meet Cortez from Rhino Squad. Gather some ammunition and walk trough the red gate. Push the button to finish the first level.