It's been so long, but I believe I learned how to do it watching YouTube videos on the subject. They will take you through the process step by step.

Here is a link to Thunder Store to choose and get mods (Don't be worried. They're free even though They call it a "store".)
https://valheim.thunderstore.io/?page=1
I'd recommend you hit the tab labeled, "Top Rated" to start.

Be warned. Mods can radically change the game by making it too easy or too hard. Plus they can break your game. You can lose valuable equipment, progress or even whole saves of your character or world, especially when updates are not compatible with your mods.