Nothing you can't live without. I'd suggest you read your skills as you acquire them - some require you attack from the flank or back to produce their full effects, so you'll want to do that whenever possible. Good tanks will always turn the enemies so you have a clear access to the backs/sides once you get to 15 and start doing multiplayer dungeons.

Till then, just have fun and don't try to rush the content. There's pretty much a tutorial for everything in the game in one way or another so most of your questions will get answered by just playing. One thing to note is that to unlock Dragoon later on you'll have to level the Marauder class to 15, so you may want to get started on that sooner rather than later. You in no way have to before you reach Lancer level 30, but the cross class skills can come in handy. Just like picking up Cure or Physics from one of the healers.

Otherwise, if you have any specific questions, feel free to post them here.