The reason you may find conflicting information is because what is required is actually different from city to city and state to state.
Best way to find out what is required in your city is to take a trip into your local chamber of commerce.
I started my first business when I was 15, and that's how I got started. Just walked in and asked. What was required then in that city was different than what was required 10 years later when I started another business.
This thread should help narrow things down some:
Business License California
And this one:
What is needed to legally sell T-shirts online
This thread should also help some:
Any advice for a 16 year old starting out in the t-shirt business?