I use a program called SketchUp (now owned by Google) for basic design work. They have a free version of the software that will more or less cover everything you need. It's very easy to use relative to other programs like CAD, though there is still a learning curve. They have a few introductory tutorials to get you started, and after a few hours of practice you can probably get the hang of it.

What's nice about it is that once you have the basic room model, you can use their 3D warehouse for all the other stuff like furniture, cabinets, lighting, etc. There are even some companies (i.e. Pottery Barn) that post all of their product in the 3D warehouse for free. So once you have the design finished, you can go purchase the exact item from your model.

Here's an example: