How do I host a server?

Hello, I would really like to host a Garry’s Mod server.
Problem is - I don’t know.
When I host in-game Garry’s Mod, people can’t connect.
I heard you can connect in Source Dedicated Server, but i can’t find it in tools.
Can anyone help?

The reason they can’t connect is because you need to forward your ports if you’re behind a router.

how do you forward ports and what one do you forward

UDP 1200 (Friends Network)
UDP 27000 to 27015 (Gameport)
UDP 27020
TCP 27030 to 27039
TCP 27015 (SRCDS Rcon port)