how do i change my main class

[Izularia]

Once you reach level 10:
-Go to the City which is home to said class you want to change to.
-Talk to the receptionist of said class' guild twice.
-Complete the quest.
-BINGO

I don't think that's what they have asked.

OP, go to your Control Panel and then to "Edit Character", there you can set your main class.

[KoroushSensei]

where is this control panel? i checked forum and ffxiv character settings and nothing

[SpiritMuse]

At the top right of the screen next to the big blue "Log out" button it'll say "Welcome" and then your name and underneath that "Settings" and "My Profile". Click "Settings". In the next page there'll be a column on the left side with, among other things, "Edit Character". Click that, and that'll take you to the page where you can select which character you want to display (if you have more than one) and what classes or jobs you want to show.

Hope that helps.