Once you have a topic, start looking for works on your subject. A mixture of articles and books can be useful, depending on the subject and time period:
Generally, books tend to be more influential and widely referenced than articles for most older subjects.
For more modern subjects, articles will be more available, but books will still cover more ground than most periodical articles.
Why consult an encyclopedia or dictionary? Scholarly subject encyclopedias and dictionaries are excellent starting places for researchers just beginning a project. For one, they can provide contextualization for any number of topics. They can also provide helpful key terms that can be employed in other searches. Fortunately, many subject encyclopedias and dictionaries provide bibliographies for these entries. These bibliographies can be invaluable in pointing you to important resources on your topic.