This is a really crappy mockup of how I imagine the main menu looking. Title on the bottom, start button that takes you straight into the game (unless we have modes or a level select, in which case it goes there), and the obligatory exit button that gets you out of the application. What other paths do we need? I figured the tutorial would be in-game as the first level or wave or something like that, but maybe a help button? An options button? If we have an options menu, what kind of options do we need?
Awful mockup

first menu mockup

edit: why the hell does wikia have achievements, that's so stupid