Find a place where you can build a longhouse. Cut down some trees to gather wood. Use the wood to build the longhouse.
Inside you’ll find a place where you can craft items. Stand there and hold E until you see the crafting recipes. Find the recipe for a raft. It’ll tell you how many pieces of wood and how much resin you need. Go out and cut down more trees to collect these. Then come back to the crafting area, and press the button to create a raft.
I’ll be put in your inventory, probably in the quick bar somewhere. If it’s not in your quick bar, put it there.
Now go to the shore and press the button on the quick bar for the raft. You’ll be asked to position the raft and press F. Do so. Now you have a raft.