SketchUp is a great tool for 3D modelling or block modeling. I use it all the time to work up plan drawings to show clients but also as a checker for myself to make sure the garden flows the way I want it to, think it will. That focal points and views framers don’t do ‘odd things’ from other angles. I learned most of the basics at College but a really excellent starter course can be found at http://www.lynda.com/SketchUp-tutorials/SketchUp-2015-Essential-Training/188209-2.html which gives you enough of the basics to get started and building your own 3D models to experiment with.
Another good aide memoire for SketchUp outdoor spaces is http://landarchs.com/top-10-hints-tips-sketchup/
I really like exporting an animation, like a short movie, so that the model can show itself off to the client on presentation day. I’ve uploaded a few of them to my YouTube channel to give yo an idea. This one embedded below, is a garden designed 2 years ago and going from strength to strength. It’s going to be seen by quite a few more people this weekend when the house builder/developer brings some of their clients along, mainly to show of the the wonderful house but I’m sure hat it is set in a beautiful garden does a bit to boost the look!