Dedicated servers are servers that stay up on your computer for as long as the PC is running.
I suggest you don't host one unless you have a spare PC or really good specs on your current PC.
If you want to know why, my PC's power supply was blown out after my parents turned it off in fear that it might overheat. And I had a Dedi up that day.
But if you want to set one up-
1. Select the addons you want for your server.
2. Open the blockland folder and look for the-crap, its....someone post below me what file you are supposed to click
3. start a single player game, and exit out of blockland once you have spawned.
4. Click that file....crap. Still can't remember.