You will want to lvl your whitemage to lvl 18.
When you have done that then you want to unlock the sub job quest and choose a sub. Since Blm is at this point available, go with it. So while lvling blm to become your "Temp" sub, switch whm to sub and blm to main.

After you get your blm to at least 15 then you can switch back to your Whm as a main job and Blm as the sub and lvl to 30.

When you get your Whm to lvl 30 you will finaly be able to quest to unlock the smn job.

After this is complete you would probably want to use smn as the main job and whm as your back up or sub.

The reason this is, is because your sub job no matter what it is will only function at half (and this is at max) of what your main job is.