If you picked a topic with which you are unfamiliar, you can probably find an overview in one of these resources (it's a good idea to know a little something* about the topic before you look for monographs and scholarly articles):
* 1-min rule: You can talk about your topic for 1 minute uninterrupted, without consulting outside sources or repeating yourself.
When you know a bit about your topic and are ready to really dive in....
Find scholarly monographs and other books concerning a single topic
Find scholarly journal articles
A quick word about search engines (i.e. Google) vs. library databases
The search box looks the same, but the underlying structure is different. Google and it's brethren are easy-going, they'll take what you give them and return something fairly relevant. Library databases (the library catalog, Historical Abstracts, etc.) are picky. They want you to be brief and choose your search terms carefully.
Tips for specific resources