There's not a huge amount of advice to give really. Jump into the water and start gathering stuff for your survival
If you're playing in Survival mode, you'll want to keep an eye on your food and water situation, and you'll get warnings when your oxygen is low. You replenish it when you swim to the surface.
Your life pod has a fabricator unit on the wall which you use to make everything you need to survive, including food and water that can be made from the fish around you. Once you've got the basics, you'll need a knife to cut things up for certain items, and a data scanner to get new designs etc. You get these new designs by scanning fragments of items on the debris you come across. Don't worry if you don't understand that now. It'll become obvious once you start exploring
The ocean contains everything you need to get started and to survive. As you start going deeper (when you have access to special items that'll help) you start to find different resources that you'll need to advance.
Your radio also gives you locations of other lifepods, but you'll need to repair it first
It's worth going to these locations as you'll get useful things at each
And finally, fear the Reaper!
Have fun exploring. It's a brilliant game!