In a perfect world, you would have time to investigate a multitude of topics until you found just the right one, a topic that held your interest, allowed you to say something new on the subject, and provided you with just enough resources to cover the material well.
But summer semester is not a perfect world. I hope you will find your topic interesting, relevant, and with enough source material. However, you will not have time to try several out. Time is short, and the process is long. So let’s get a thesis.
Now you may have your topic, but still not have a thesis. A topic is a subject, such as birth control for dogs or happiness. A thesis is the position you take on that subject. Therefore, it must be a sentence. It must be a statement, not a question. And it must be arguable. (That is, someone could take the opposite position on the topic from you.)
There are birth control options for dogs. (Not a thesis. All you have to do is see if there are any, and the paper is over.)
All dog food should contain ingredients that sterilize dogs. (Now, that is presents an argument.)
Self-help books often focus on achieving happiness. (Not a thesis statement.)
Self-help books that focus on happiness usually do more harm than good. (This is an argument.)
And you can focus your thesis even more. (Self-help books that focus on happiness usually do more harm than good by depicting a state that most people can’t achieve, by having people read about unhappiness instead of doing anything about it, and by assuming that being happy is something that can be manipulated.)
Once you have your thesis, working on your sources is much easier!