The game is in the idea of minecraft alpha (currently). Originally Minecraft (during alpha) didn't really tell you a darn thing. You had to figure out recipes, etc.

This game does the same thing. In the spirit of exploration and self reliance you're supposed to figure things out. Many end up on forms (like you asking questions), youtube videos, twitch streamers, and the Valheim wiki walkthrough site.

You're question is also not specific. You build a hammer and if you have wood or other materials can 'mend'/'fix' various abandoned buildings. You can repair tools at the workbench, provide you build one with a hammer and the proper materials.