Deploy the Sentry Turret to Defend Operations | Mission 4: No Hope in Hadley's A: CM Guide

With the turret, head for the operations room. There shouldn't be any more aliens on your way, but if you happen upon a lost soul, drop the turret for a moment so that you could reach for your gun. In the operations room, place the turret where the screenshot shows, which is by the weapons locker (take a minute to replenish your ammo supplies).

My recommendation now is to place yourself at the console that Bishop is using; this way you'll have a narrow passageway separating you from the aliens. Focus primarily on the Xenomorphs in closer range.