I want to start a website that sells posters that I create. What kind of copyrights do I need, or do I need any at all?
If you use art in the posters, you will need a license that allows you to resell the art as part of your product. There are many stock photo websites that allow this. If you are asking how to protect your products, then a copyright exists from the moment that you create your product and put it into a "fixed form", however you will not receive the highest level of protection or the ability to receive a high judgement for infringements against your work unless you register the posters with the copyright office. You can register a visual arts work here: copyright.gov/register/visual.html