You can't make your own server. You can set up a mod of your own choice locally for single player only.
You can buy your own server if you are the leader of a clan for 50k toricredits
You can buy a private server for personal use for 75k toricredits
You can build your own mods by visiting the MODS subforum and reading the stickies there.
Hi. I used to be a damn good mod (wayback 2008 and earlier) and then veb made me not a mod.
Been playing competitive Team Fortress 2.