Best thing to get first is more character slots so you can try out the other classes. It's nice to have one of every class on both sides as you never know when you may want to be a KDF Sci or Fed Eng. Multiple characters will also help earn more dilithium and that can be turned into more Zen points as you will need 2500 for any of the account wide level 50 ship unlocks. The increased EC cap is also a useful unlock early on since it is account wide too.
I would avoid getting any of the bank, account bank or inventory slot increases as if you really want those unlocks, it is best to find a cheap copy of the game on Amazon or Ebay and use the code to get Gold for a month and get all those bank and inventory slot increases for free from the Gold membership. That will also give you 500 zen too for being gold for a month.
I just bought a $10 card, bought keys and played the tholian lockbox lottery with them. I got 3 duty officer packs and some ??Fleet Credit Bonus?? things. I'm not going to do that again although it seems allright that I got some more duty officers.
Quick question, are you silver or gold player, if silver you should subscribe for 1 month to level up and get all the perks and then use some of your card or the 1 month stippend to get the EC upgrade - or find a very cheap copy of the game for the 1 month gold
My advice would be to hold off spending it straight away. You will be wanting to research the game first of all, get a feel for it and you will see more clearly where you will want to spend that money. Everyone plays differently and in a few weeks you will see how you own game play style develops and you will have a better understanding where that money will be better spent.