How should I start in robotics?
I want to answer this question; really I do. I honestly can't, though. The only true answer I have is "read". That's how I started, that's how I have continued to learn. Robotics (which at once is both a sub-field and super-field of computer science, IMHO), I have learned, encompasses so much - that I find I can't think of a place to start, other than by reading.
I would say as far as hardware is concerned, buying something now might be premature - with the exception of things like a Lego NXT or Mindstorms kit, or perhaps (if you have the money) a FischerTechnik kit. Basically, if you really want to delve into the mechanics coupled with the programming of robotics, you need to go at it "open-ended"; you don't necessarily want a single small robot. In many cases, you can learn a lot about robotics without any robot at all, by simulating a robot on the computer (in fact, there are several good 3D robotics simulation systems available for most PC platforms that you might want to look into).
You could really start with an Arduino and the blink sketch; understand that, then move on to controlling and getting input from other things. A switch becomes a touch sensor, a motor controlled by a transistor a "muscle", and the Arduino with its programming, becomes the "brain" of the robot. Attach it all together with some hot glue and double-sided tape and you have a desktop bump-switch rover-bot.
But ultimately, reading and studying robots, robotics, computer science, and a whole vast host of other subjects, while taking notes (physically or mentally - whichever is best for you), asking questions, and seeking answers - that's how you pursue it. Robotics, ultimately, is the study of the creation of artificial life and intelligence. You may as well be asking "How should I start in religion?" (indeed, you study enough of robotics, the concepts of religion do come into play!); there is no one "right" answer, no one possible method or approach. You just have to begin, and one thing will lead to another, and at some point you look around and wonder "How did I get here?"...
That's what I'm doing right now - your question has caused me to pause and wonder just that, because I first approached the question with the idea of "how did I start?". I found that I mainly started with reading, studying, dreaming, drawing, conceptualizing, thinking, wondering, doing. Some things didn't work, others did; but no matter what, I have just as many questions today as I did when I started thinking about this field when I was 6 or 7 years old.
I am sorry that I can't answer this better - really, this forum isn't the place to give you a proper answer; but I don't know what is...
:-/