Your teacher will often give you at least a broad topic range for your paper. Narrow your options down to what you consider to be most interesting and arguable. For example, if you are writing a term paper for a class on the Vietnam War, you may want to explore the reasons behind President Nixon's invasion of Laos. Do not pick a topic too broad because it can lead to vagueness in the paper and a lack of a cohesive thesis.
Once you have narrowed down your topic area, start researching. The key here is to first gain as much knowledge as you can about the topic area, and gather all the resources you will need for your essay. For example, if you are writing about the bombing of Pearl Harbor in 1941, then anything from first-hand soldier accounts to historical books will help.
Most papers require a thesis. If it does not, like a paper surveying folk religion in China, list out similarities and differences between the various religions. Pick a topic you like and that your teacher will like. Most teachers are willing to hear paper ideas beforehand. In advanced college classes, it is important to stray from obvious, commonly argued points. For example, arguing why slavery is immoral is a valid, important point to make, but will not impress a college Humanities professor. Making a unique sociological, economic, and political based thesis for why modern sex slave systems exist in the world is far more interesting.
Figure out what parts of your books and other resources you will need to use. Write out a thesis, if your paper requires one. If your paper is not arguing a point, write out an abstract detailing what your paper aims to cover. From here you are ready to begin.