Lovely quilt!
The most important thing I learned when starting to machine quilt was, you have to go easy on yourself and not have expectations of perfection. Excellent idea to do some practice first!
Personally, I'd start with something simple! You'll need some kind of guidance system if you're going for straight line quilting (drawing lines, using tape, etc). I would suggest starting with some slightly wavy lines, which you could do free-form, placed somewhere within your larger squares. I would stay away from stitch in the ditch, it's not that easy. Doing outline quilting (a short distance away from the seam) is easier than in the ditch, imo.