If your goal is to design, not just to practice with SketchUp, don't simply take an existing plan, make a few changes, and call it good. Instead, think about the qualities that make for a great house, and incorporate those qualities in your design.
Christopher Alexander and his colleagues thought about the qualities of great design, and in 1977 they published their thoughts in "A Pattern Language." The book is widely available, but it is a little pricey. Even if money is a problem for you, though, you are in luck: Alexander has a web site.
http://www.patternlanguage.com/leveltwo/patterns.htm
You might not agree with some of the conclusions, but I guarantee it will get you thinking.
Also, it would be good to study the works of past masters. (Learn, don't worship.) You could start with Frank Lloyd Wright. Both his work and his biography are amazing - so much so that I sometimes wonder if he made a deal with the Devil to become the best architect ever. You can find a lot about him on the web, and I would be surprised if there weren't books about him at a library near you.
If you do become an architect, I wish you great clients with great commissions and the talent to do them justice.