Really, you just pick one.

Look back at your studies so far. What voluntary classes did you take? When you got to pick a subject for a paper, what did you write about? What do you read about? For me, this largely came down to the history of the socialist movement, media history and Sub-Saharan Africa during the Cold War. So I continued to focus on those while getting my Master's degree - but the real focus came when I had to pick a subject for my thesis. Having narrowed it down somewhat, I started thinking about possible subjects and began foraging around. Talking to other people, reading articles, fleshing out some vague ideas.

In the end, I lucked out. I'd been wanting to write about something newspaper related. I'd just read Nick Davies' Flat Earth News and was kinda mulling over writing about newspapers (and news) within capitalism. At the start of the year, our faculty basically gave us a list of suggested topics for our thesis - in case you weren't feeling particularly inspired. It also included a list of newly available archives. One of those just happened to be of a socialist newspaper that went bankrupt during the seventies. Not only was it a perfect case study, it also combined two of my preferred subjects in a single topic.

But you don't have to rely on luck. You seem to have some vague ideas about subjects that interest you - the JFK era, French history. That's already a great start. You could start looking for something that involves them both? The post-war and then JFK era in France was absolutely fascinating - the Algerian independence struggle, the political unrest/near-coups, the (student) protests, the flirting with authoritarianism, and so on. At the very least, thinking about the possibilities of a cross-over might help you figure out just what it is you like about each subject.

It also helps to think about what topics you're not interested in, or at least which ones you don't find quite interesting enough to focus on. For example, military history bores me to tears, so I can easily rule that out as a focus. Since you've graduated with a degree in History, you've obviously looked at a variety of times and places in world history, so you could think about which of those you didn't find very compelling, and why, to narrow down your choices.