but yeah i guess i should type up at least a bit more than that
@Nite1
that first 95% of the game i mentioned, largely the best thing you can do is put together what used to be known as a "Holy Trinity" team and just go to town -- the Holy Trinity being a combination of
1. Boostga/Faithga Soul Break (generic terms the Reddit community has given to +ATK/+MAG Soul Breaks

oh yeah and speaking of Reddit the one for this game is insanely good and loaded with information, find the FAQ .PDF!
http://www.reddit.com/ffrecordkeeper
SBs that give party haste used to be included here before Hastega existed as an ability
2. Mitigation (Protectga+Shellga+"Wall" (Wall = SB with combination +DEF&RES see:
Equipment_Rarity 6_Divine Veil Grimoire currently known as "Godwall" because it combines all three of these PLUS Hastega!

3. Group Heal Soul Break (preferably an instant cast USB heal)
for most of the content you'll be doing, tho, the Roaming Warrior (RW) system can easily replace one or two of these. "Godwall"/Divine Veil Grimoire that i mentioned is one of the best RWs since it saves so many "slots" here. also really good is Hyper Mighty G, Rikku's USB which switches the +DEF/RES "Wall" portion of Godwall fora combination +ATK&MAG.
also super important in this portion of the game are abilities and soul breaks that debuff the enemy. first are the 3* "Break" abilities in the Combat school, then the 4* "Breakdown" abilities in the Support school. finally there is the 5* Support ability Full Break which is like all the 4* Breaks glued together AND also stacks with them, meaning you get benefit from using both still.
then of course you need abilities and soul breaks that just straight up do damage, but that portion of the game is a lot easier to figure out. basically whoever you get a good Ultra or Awakening Soul Break for, focus on abilities that they can use. the Soul Breaks you have available to you almost completely dictate what damage abilities you should be creating, it is hard to give generic advice here as it is with the other abilities. you should bring Full Break and either Power Breakdown or Magic Breakdown to every fight (Power or Magic depending on what types of attacks the boss uses mainly), if not BOTH Power and Magic.
so yeah, your first goal should be getting one set of 5 characters to level 99, broken down roughly like:
Damage dealer
Damage dealer
Damage dealer
Support
Heal
depending on the Soul Breaks you get you may end up running a character that is more of a combination Damage+Support for some of your damage slots.
your next goal alongside getting a group to level 99 is to put together the abilities needed to beat the Nightmare dungeons
reddit: the front page of the internet you used to be able to beat these well before level 99, but they do require specific abilities to be able to do, so i'm unsure how quickly you can get to them starting fresh today.
unlock those and you can start doing Magicite dungeons. Support abilities don't work in the first two difficulties of Magicite dungeons, which is why some people will tell you that Support abilities don't matter much anymore. they absolutely do still matter for you, and will for the majority of the content you have available to you, i'm not sure why so many veterans give this particular bit of advice to newbies. Support abilities even start working again at the 5* difficulty level, meaning there's a total of 14 bosses where you don't want to bring Support.
i think that's the general outline for how you'll spend your first few months at least. crack away at Realm dungeons, level up, make abilities, and ask us which banners are good to pull on!

tho if you want to spend your mythril on your favorite characters / games instead do it, i'm always a proponent of fanboy pulls.