I agree with Dan on that point! Find out what you need for your pet, before you get your pet! Spiny Tailed Iguanas, well, Iguanas in general are not the easiest reptiles to keep and they are not a good beginner herp.
<Getting off the soapbox now>
I have answered all your questions in the chat already, but here it goes again:
Please feed your Spiny Tail a mixture of chopped up greens, fruit, and canned dog food on a daily basis. At good ratio would be 50% greens, 20% fruit and 30% dog food. Mix well, or the spiny tail will pick out his favorite food items and not get a balanced diet.
Use greens like collards, dandelion, bok choy. DO NOT use greens such as ice berg lettuce and romaine. They do not have have nutritional value for your pet. Good fruits to use are melons, strawberries, apple, grapes, mango.
Also make sure that your husbandry is correct.... do you have a UV light for the spiny tail? Is the enclosure hot enough? A place to climb? etc etc etc
If you haven't already PLEASE check out Melissa Kaplans site on Iguanas.
http://www.anapsid.org
Cheers!
Trace