I have this script in my home folder (pi): server.py.
How could I make Debian run this script at startup so I dont need to log in and to i manually?
By default debian uses SysV init scripts with the start-stop-daemon binary. There is an example script, /etc/init.d/skeleton you can use as a base for your init script. Once you have your init script and it is marked as executable, the update-rc.d command can be used to add it to desired run levels.