Get a USA guide to usa stamps the us postal service has them or go to
ebay and buy one it could be a 2014 guide or other older years whatever your price range is comfortable spending on it. Some of the usa washington/franklin head stamps you might want to see if they are watermarked, I don't know how we can tell that by sitting on our end, you will see some of those have a higher value then other unwatermarked.
Here is a worldwide stamp guide online it is free to signup for and when you click on the catalogue button on the left side it has the whole world to look up.
http://www.stampworld.com/en/Good luck I have had fun flipping through both guides, they are very helpful.