How do I set a task to run every so often?

/Library/LaunchDaemons/

/Library/LaunchAgents/

~/Library/LaunchAgents/

com.example.my-fancy-task.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.example.my-fancy-task</string>
    <key>OnDemand</key>
    <true/>
    <key>ProgramArguments</key>
    <array>
    	<string>/bin/sh</string>
    	<string>/usr/local/bin/my-script.sh</string>
    </array>
    <key>StartInterval</key>
    <integer>1800</integer>
</dict>
</plist>

launchctl <command> <parameter>