Can anyone tell me how to use the src on a server cause when I use it and type ./start.sh, it tells me Composer autoloader not found. I use CentOS 7 btw.
PocketMine uses Composer for automatic dependency managment. You need to install Composer to run it from source, which you can do on CentOS by following these instructions.
Yes, either install Composer as root or use sudo. But make sure to switch back to a regular account before starting PocketMine.