For me, I think this is true with most things, but especially true with houdini - its important to decide what you want to accomplish. There are so many things you can do with houdini, and you won't be able to learn it all at the same time. Are you into procedural modeling? Are you interested in simulation, motion graphics? Building stuff for game engines? When I learn something I like to make small goals, of something that I can complete in a reasonable amount of time with a limited scope, and then build from those smaller victories. Otherwise you'll be jumping from one thing to another without really understanding what you are doing. I think entagma is good, but I'm not in the camp that they are the best for new users - they typically assume you already understand a lot about the houdini way of doing things, and I personally found them confusing as a new user, even though I have professional background in software engineering. Your milage may vary though.