What should I focus on doing first?

If you are familiar with this type of survival game you will already have some idea of what you need to do as you explore the island. You are basically looking for and collecting resources that you can use to craft items

To begin with you will find flint and tree branches readily available on the floor which you can pick up to make crude tools that can then be used to chop down trees and mine rocks. This will enable you to craft better tools which you can then use to hunt animals for clothing and materials. You will also need to collect plants and herbs so you are able to craft medicine if you are poisoned and wood to craft splints just in case you have a high fall and break a limb.

The game itself will teach you how to craft these items as well as others you may need. What you need to do to give yourself the best chance of surviving is to make sure you are properly prepared for the treacherous adventure that awaits you.