Until level 50 I would just use the vendors to fill in any holes in your current gear score. At level 50 you will get a set of ilvl 90 left side gear by completing your first class quest.You get a piece of useful left side gear by completing the dungeons in Stormblood and there are also numerous chest in each one so gearing up 61+ won't even be an issue.

If you are doing dungeons of your current level do not give in to people who ask you to do big pulls. One pack at a time is fine for leveling dungeons. I see tanks over-pulling dungeons all the time because they feel pressured to pull like someone who outgears it and is synced.

In leveling dungeons keeping threat, using your cooldowns on boss abilities and packs of trash, and following mechanics are the most important things right after fun.