How do I come up with a dissertation topic early on in my degree?
This is a classic example where talking to your advisor is of the utmost importance.  Some advisors want their students to choose a topic as early as possible, and others don't mind if their students take their time. As for the topic, hopefully, your advisor should be able to point you in a direction if you don't already have one.

While I understand the rush to get started on something concrete, you really don't want to do a lot of work on a topic that ends up being (1) uninteresting, and (2) unfruitful.  Talk to as many other faculty and students as you'd like, and there isn't anything wrong with reading ahead, with the exception that you might not understand something well enough to make a good decision about whether you should do research in the subject.

My bottom-line suggestion is to take your time picking a topic, because you really do want it to be a good one.