1. Choose a broad subject and do some preliminary research. Read an encyclopedia article to get an idea of the scope of the topic and ideas on how you might choose a narrower focus. For example, if I want to study fabrics, a general article will help me focus on one fabric (linen) or one technology (weaving) or one social aspect (weavers). You can find good subject encyclopedias in the library catalog by entering a search that includes the word "encyclopedia"; for example: egypt encyclopedia. Try the database Gale Virtual Reference Library, which has scholarly encyclopedias for many subjects.
2. Do more initial research on your narrower topic. See how much information is available. If there is a lot of material, you will need to focus your topic further so that you can manage to read some of the most important research in that area. The materials available will help you narrow your focus. (Don't narrow your focus and then look for materials - you might find way too much or not enough.) For example, I may now want to narrow my topic to The Use of Linen for Sails in Ancient Ships.
3. Write a good working thesis and a good outline. If you (and your professor) are happy with the thesis and outline, stick to them. When you find interesting material that doesn't fit, don't use it! Don't go off on tangents (it's easy to do, since ancient history is so full of fascinating stuff!). When you find a good article, ask yourself, "Where does this fit in my outline? Which major point does this explain or support?" For example, if I find a wonderful article about weavers' guilds in ancient Egypt, I'm going to have to leave it out, since it doesn't deal with my topic of linen sails.