nexusmods is the largest and safest place for valheim mods, and as for which mods, it depends on you, think what you want to change and search for that keyword.

for example you don't like how forsaken powers work or want a change in them, search forsaken and look through the mods it comes up in site search

Nexusmods is definitly the right place to start. Everythign you will need to get started in moding is right there.

I've been using the BepInEx mod engine for a few months and it has really saved the game for me. It's easy install, download package for your OS, copy contents of zip into Valhiem game folder and your done. be it Linux or Windows. Don't like BepInEx? delete the BepInEX folder from the game folder and it's gone. It doesn't change any of the games native files at all.

Some mods that I have found to be useful are Quickconnect, BlastFurnaceFix, and BetterExploration. Quickconnect adds a side menu to the main screen to connect to servers you like. BlastFurnaceFix lets the blast furnace process Tin and other low tier metals, something the devs will likely do eventually anyway. And betterExploration extends the map reveal distance closer to actual in game visual range, the entire area of the mini map can get uncovered if you get to a high spot. This mod does add a little extra lag to the game sometimes when it updates the minimap when you hit a high spot but not really that bad.

As with any changes to the game folder be sure to make a backup of it before doing anything. I've yet to hear of anyone having a problem with Valheim mods but always better to be safe.