Coot, you're just scratching the surface of things. I won't say more as this game has spoilers galore. Keep trying to get PDA's. Explore all the beacons that get displayed after you receive radio messages. If there are no more radio messages left and you're stuck with nothing left to do, go deeper..never stop going deeper. Won't say anything more. ...
As Longbow says, PDAs and radio messages are the main tools driving the story for a while. On top of that, random exploring may take you to various sites or apparent dead ends where you can't get any further. It might be worth having some beacons handy to drop (and give reasonable names - that's possible!) at such locations to find them again later There is also an option to manage which signals are shown in your nav panel (or whatever it was called…) to prevent cluttering.
