There's not too much to say about lower levels, as most of it has been streamlined. Most EXP related things (outside of Abyssea) happen in GoV. You should solo to Lv12+ in the starter aeras, then move to Gusgen Mines to join a GoV party. Just go in with your flag up and ask people pting there to team up. You get a certain "page" from the Grounds Tome nearby, which instructs you to kill a certain mob x number of times. Then all you do is keep killing these mobs, and every few kills you'll get a big EXP bonus. You can easily stay there until Lv30 or higher, and at some point you could move to other GoV locations (Crawlers' Nest is popular for the midlevels). Your skills may be gimped through all the syncing, but it won't take too long to catch up, especially with some gear and DRG's innate accuracy bonus.

As for subjobs, /SAM is not very useful before you get Hasso and Meditate, so 50+. Before then /WAR will do just fine (as will most subs, since the abilities you get from subjobs before Lv20 are rarely of much use), but afterwards you should try to get /SAM leveled up after all.