www.greenigsociety.org
www.iguanaden.com
www.anapsid.org
"Iguanas for Dummies" by Melissa Kaplan
"Green Iguana: the Ultimate Owner's Manual" by James Hatfield III
There is just way too much ground to cover in a forum like this. Generally speaking, you want to read as much as you can and then come back if you have any more specific questions. Read through the information on those sites, or print it out and take it home if you have to.
With regards to the books, it is widely accepted that those are the only two worth owning. I recommend getting both of them. The second is a little pricey for a pet care book, but it's worth it in the long run.
As far as the questions you asked:
He'll outgrow the 20 gallon tank within 6 months, so you should definitely be thinking about the long run and have some plan in the works for a bigger cage. You can buy one at a place like www.cagesbydesign.com, but most of us simply build them ourselves.
What are you using for substrate (that stuff on the bottom of the tank)? Iguanas should not be kept on anything particulate, like wood chips or sand. This is because they can easily ingest it and it can cause numerous digestive problems.
You mention having a UV light. By that I hope you mean UVB. It will be a fluorescent light that puts off almost no heat and a brand like Reptisun or Reptiglow that says something like "5% UVB" right on the packaging and even on the bulb itself. Make sure that there is no screening or glass between the bulb and the ig, or all the good UVB is being filtered out.
The spotlight sounds good... if you know the temperature it is warming the cage to. If you don't already, make sure you get a good digital thermometer. They sell some that come with two probes, so you can stick one under the basking spot and one on the cooler side of the cage. Correct temperatures are vital.
You don't mention humidity. Iguanas need 60-80% humidity for their sensitive respiratory systems and also to ease shedding. Without it, they can develop respiratory illnesses and their skin will slowly dry as they age. The biggest effect you'll probably see is that he'll lose all his beautiful spikes. To monitor humidity, you'll need a hydrometer. Again, it pays to go digital.
You say you've got food, but don't mention what you're feeding. With green iguanas, it is very important that they have a wide variety of fresh food available to them each and every day. Supplements are also important. I won't go into too much detail on that, though. It's a pretty deep subject, and you'll find lots more information in those sources I posted.
If you have any more questions, be sure to ask! And welcome aboard!

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~Alika~
1.0.0 green iguanas
0.1.0 cockatiels
1.0.0 senegal parrots
0.1.0 blue-fronted amazons
0.0.1 red belly piranha