I was wondering how far I can upgrade my Skyforge Steel Greatsword? Can I only get it to exquisite?

With the Steel Smiting perk you can get it to Legendary, though this will require a Smiting Skill of 91.

Should be able to get it to legendary. I dual weild two skyforge steel swords, so the greatsword should be no different. In fact, as far as I know, the only things that can't be upgraded to legendary are daedric artifacts, which gets stuck at flawless, which is better than exquisite.

Where is the steel smithing perk? I can only see one steel smithing perk which I already have. Or once I reach level 91 I will I automatically be able to upgrade it by going to a smith?

He is saying that at a smithing skill at level 91 with the steel smithing perk, you'll be able to improve it to legendary. You can upgrade it at anytime, but not as much without those two things.

Once you have the perk your skill level will determine how much you can improve an item.
http://www.uesp.net/wiki/Skyrim:Smithing#Item_Quality Most of the information you'll need can be found in this link.