"Find a Way Out of the Sewers" is a mission objective in the game Aliens: Colonial Marines
You need to find a way out of the Sewers
Look for pumps to turn. You will have to activate several of those to do stuff such as Turn on Sewage Pumps and Open Sewer Bulkheads. The pumps that you need to turn will be marked with yellow exclamation marks.
Soon you will encounter Boilers, which are Xenos that can explode into acid. They cannot see but they can hear you move, so move very slowly, and if a Boiler is near you, stand still until it goes away. Use pumps to activate fans in the sewer which will create noise - the Boilers will go and investigate and then explode. That will clear areas of those boilers.
Once you've cleared enough of the Boilers, look for some stairs onto a raised metal platform. There is a door there, looks like this:
You will have to cut the door open, then go through the door and walk some more until you reach the next objective and see O'Neal