First, make sure it holds water, obviously lol. 55g tanks arent very expensive new, Id say around $70. So the last thing you want is to ruin what Id imagine is going to be near $700 worth of goods, live stock, etc, because the free tank leaked. And also, clean the tank with vinegar, if you have not done so yet. To get the crap from the dirty reptiles out.
For your lights, I run the T5 Nova Extremes, 24" 4x24w on my 20g tank. And I love them. I Wouldnt worry too much about the upgrade of the reflectors just yet. Thats in due time Id imagine.
As for a skimmer, you should go with something to hang on the tank, since youre new. Im still new, and I think its too in depth right now to run a sump, skimmer, etc. CPR and
AquaC Remora Pros are good Hang On Back (HOB) skimmers, from what Ive read. I run a CPR, and its great, though its a little picky with where the collection cup needs to sit.
MAke sure your sand is "new", and fine enough to get the anaerobic effect for some bacteria under the top layer. Also, you will need live rock. People say its typically about 1lb of LR per Gallon. So with your filter (Which you may not need) and your skimmer, Id say 50-60lbs of Live Rock is a safe bet. Or if you want to go the cheaper way, get one nice piece of live rock, and then get some base dried rock, and let that become live, though that will take way longer.
Your tank has to cycle, yada yada yada, get some tests, yada yada yada. You need a clean up crew after this is done. Probably about 40-60 crabs, and Id say about 10-20 snails. After that, get a cheap ugly fish, like a damsel, to see if you kill him, instead of any other fish that you like and are more pricey lol.
I Think that about sums it up. You can read through the archives, and you'll find tons of info. I dont know too much still compared to a lot of people on here, but before I Came here I didnt know ANYTHING.