It's really simple to use, which is why it is so powerful.
The most important thing to do is to keep your parts separate by creating components. Once you're done with a wall, triple-click it, then right click and click create component. That way, all the drawing tools and push/pull won't affect that part anymore!
I've used Sketchup and CAD for work quite a lot, so if you get stuck or need help figuring something out, feel free to ask.
