Visa questions as an American.

I am planning a trip to Thailand for the end of the year. November/December I'm still shopping around looking at exact dates. I'm looking up Visa information but I keep seeing conflicting information. I would like to stay 30 days. But some places I see I get a 15 day Visa I can Extend and others I see a 30 day visa I can extent. Which one is it? Also does this mean I have to have a 15 day exit ticket and I would have to buy another ticket if my visa is extended?