Spend a lot of time riding your bike during the summer to build up a nice base. Once cross season approaches, you are going to want to ramp up your training and do some higher intensity-type intervals. Start in the beginners race (C race or Cat 4 depending on what your state calls it). Don't be afraid to jump right in and try a race. The people are fun, racing is entertaining and it doesn't matter how well you do or don't do. Results will come with experience and you only get the experience through racing.
You can make barriers in your backyard. Some wire clothes hangers and a little ribbon will suffice (or whatever crafty thing you can come up with). Practice dismounts and mounts. Definitely check with your local Cyclocross community and see if they hold any CX clinics. As there is a lot of technique in mounting and dismounting your bike. If you don't have access to a clinic, watch a race or two from the barrier areas. Make sure you watch the Cat5 all the way up to the Cat1,2 races. You will see a huge difference in the ability when it comes to mounting your bike and it will teach you quite a bit.
You might also want to do some actual "brick" workouts where you mix a small running in between riding. It is one thing to practice mounting and dismounting your bike, but it's completely another to do it while you are actually working out or racing. Your body will only do what you train it to do.
I find Cross to be really challenging, but super fun. I've primarily race mountain bikes and have done that for a number of years. I started racing Cyclocross in 2006 in the Cat. 3 races. I did pretty well overall (won some $$ and took home a trophy for 5th overall for the 2006 series), but it really was a struggle at the beginning....when I was bringing up the back of the pack. But then again it really depended on the course. I obviously did better on the technical courses as my mountain bike skills were helpful. I did awful on the roadie courses and got dropped like yesterday's garbage which was very humbling (and I'm and expert level mountain biker by the way).