I just started up Union X myself about 2 weeks ago, so here's my findings.
-Vulpes is the best Union so join that one.
-Eventually, you'll have to do all quests (700+) but you can join play Union Cross to level up fast! (I hit level 200 in just a couple rounds)
-Find a good party to give you good medal support
-Definitely update to 3.0.0 so you can play the Classic Kingdom games
-Save jewels wisely! You start with a nice sum, but save them for good events
-New events happen weekly (I think), but they'll be quite hard to complete as a beginner (lvl 1000+ quests are stupid hard)
-Avatar coins ARE important. They help you get Cost points, which in the long run allows you to evolve your medals. Evolved medals have a higher Cost points so be careful before you evolve.
-GEMS ARE important. You can get them from weekly events and special quests. They are needed to max out keyblades after lvl 26.
-There is not a tier list for medals apparently. Some medals are better than others despite having similar or same stats (like attack bonuses) so observe the stats closely.
And that's all I really know as a noob. I got up to the 200+ quests in 2-3 weeks. Once you hit Union Cross a bunch of times, everything is cake walk until you start getting to the 300-400 quests apparently.