If there's someone else in the guild, you can't disband the guild- if you're the guild leader, you can either kick everyone else from the guild and then disband it, or else appoint someone else as guild leader and then leave the guild. If you're not the guild leader, all you have to do is click the arrow next to your name, and there should be an option on the menu that pops up to "Leave My Guild". If you kick everyone else from the guild, if you're the only person in the guild and you click on the drop-down menu, you should have the option to "Disband My Guild".