You can collect by type or specialty, topic or composition. There is no right or wrong way. (I suggest that you collect what you like, in the highest quality you can afford. That way, if you are ever forced to sell your collection, you'll get a decent return on it. Let's hope that never happens, though, and that's just one accumulator's opinion.)
One expert has given his opinion here. He makes some good points, so check it out.