To change your server into creative mode:
Login to the Minecraft Server Control Panel and navigate to your server.
Click on the 'Console' link on the left-hand side of the page
In the text entry field type: gamemode creative playername replacing the word playername with the Minecraft username
Hit enter or click 'Send' to send the command to the server
To change yourself back to survival type: gamemode survival playername
More server commands can be found here.