How I can start mongodb automatically?
If you have to use sudo to load it, the plist file may be owned by root. Launchd will only automatically load agents that you own from ~/Library/LaunchAgents, where it will load agents owned by root from /Library/LaunchAgents.

You can do ls -l ~/Library/LaunchAgents to view the ownership of the plist. If you actually want it to run under your user account, you can change it's ownership:

sudo chown `whoami` ~/Library/LaunchAgents/homebrew.mxcl.mongodb.plist


However, if you need to run mongod as root, you should leave the plist in /Library/LaunchDaemons