Find a subject that interests you, or that links to your other interests. As you do you research, you will need to narrow your topic to be specific enough for you to cover adequately in the page length available to you, and broad enough that you can find plentiful primary and secondary source material.
You can often learn some basic information about your topic by looking in online encyclopedias, especially Encyclopaedia Britannica. BEWARE OF WIKIPEDIA! Feel free to consult Wikipedia, which is often full of valuable information and is very up to date. HOWEVER, Wikipedia's unique system of a community volunteer writers also means that you have no guarantee that the author is qualified on his subject, or has correct information. Pages have been changed or vandalized, although they are usually corrected quickly. Thus information in Wikipedia can be very unreliable. Always verify information you find there. I suggest never citing Wikipedia in your paper, but rather relying on other, more reliable sources instead.
Locate secondary sources to give you background, suggest possible thesis statements, and point to potential primary sources. "Secondary sources" are books and articles that are based on primary sources, and generally are footnoted and are not usually written by participants in the events you are writing about.
Locate potential primary sources. "Primary sources" comprise sources of many kinds: books, news articles, diaries, oral histories, interviews, letters, records, photographs, archives, and so forth. All primary sources were created by participants in the events you are researching, usually contemporaneously but sometimes after the fact. Examples of the latter include oral histories and memoirs.
Students always ask how many sources are enough. There is no standard number of sources, as the sources vary tremendously for various topics, but a good rule of thumb is three books, seven articles, and whatever primary sources you can turn up.
The best places to seek sources are the university libraries and archives. Most recent indexes can be found online, but before the late 1980s almost all indexes are bound volumes in the library. To find secondary books, go to the library Website and go to the Online Catalog. You can also get books that the library does not own using Interlibrary Loan. Click on FirstSearch and select the database WorldCat, which accesses libraries across the world. Once you have found a book that you need, you can request it from Interlibrary Loan using request forms. Books take anywhere from a couple of days to a couple weeks to arrive. Check back frequently. Articles generally are faxed and arrive very quickly.
There are also annotated bibliographies available online, which can be very helpful. For environmental history, the Forest History Society has an excellent searchable database. H-Net is an organization of hundreds of scholarly e-mail discussion lists and book reviews. The discussion logs are searchable and scholars often post queries when they are looking for sources for their research. H-Environment is the environmental history discussion list, and H-AmRel is the American religious history discussion list. H-Net is a very valuable resource.
There are many ways to find good secondary articles. Click on "Databases" to find numerous databases. FirstSearch also has useful databases to find articles, especially ArticleFirst.