Sounds like you made it this far without too much help so you should be able to make it through this bit.
To start you off, however, tip toe out of the cave and shoot the small scout then head to the end of the bridge to get your first piece of the marine mobile blueprint (there are 16 pieces to this, all in Who Lake). Now, shoot the animal you see just across the bridge and collect the thistles he drops. You use these on the scout shorts hanging on the laundry lines.