I’m no expert, but it looks like lots of people use SketchUp. There’s a free web-based version. Fine Homebuilding has some DVDs on how to use it. There are probably YouTube videos too.
And you might not like this idea, but I think you should also consider building simple models from cardboard. You can use ordinary corrugated cardboard, but chipboard and foamcore are also options. It takes time, but it gives you an understanding of a building that’s hard to get from a computer screen. You can even investigate things like how window placement will affect sightlines, or how much sunlight it will get at various times during the day. It doesn’t have to be pretty. (I studied architecture as an undergrad, but never practiced it, thank god.)