Ty a lot for the help guys!

use amp

You need to set up a port forwarding first on your router then you can proceed to next step where you can download the Valheim Dedicated Server from your Steam library it's not in the game list but it's listed as tools app, you can type valheim in your Steam library search bar and it should be appear on the list below.
But it still require you to run and let the file open to make it online, not the game but the server file it should be appears like CMD window (command prompt), if you really want to make it online all the time without you having to turn on your computer you have to pay for the dedicated server hosting.

Honestly, i personally don't think most people should be hosting dedicated servers on their personal PC unless they already know what they are doing.
Specifically because things like portforwarding and messing with router settings can cause connection issues for you or anyone on that network not to mention open you up for direct attacks. (say if a friend invited some one they'd just met or w/e)

I strongly recommend the far safer route of shelling out $5(ish) a month for a hosted server.
Me and my friends regularly alternate who pays for it. and even if you aren't home or don't have your PC on, your friends can still play.

Note that the connected game clients do all the heavy lifting and the server is mostly just managing communication between them so it's a pretty lightweight program. If you have an older computer sitting around the easiest setup is to install the server on that, give it a fixed local ip address and leave it running 24/7 wired directly to the router. With that setup you can tell the router that machine is your DMZ, the default destination for all incoming connections.

If you need to have the server running on your main device you'll need the more complex router settings shown in the guides. Eg: https://www.rockpapershotgun.com/valheim-dedicated-server-hosting-how-to-setup-dedicated-server-to-play-with-friends

Question on dedicated servers: Are there any that support the serverside simulation mod? That makes the server a lot less lightweight but could be a good performance boost for anyone connecting with older hardware.

Originally posted by Mharr:

Question on dedicated servers: Are there any that support the serverside simulation mod? That makes the server a lot less lightweight but could be a good performance boost for anyone connecting with older hardware.

I've not looked into this yet but i will email my host rn and find out.