As far as color of thread I think it depends on how much you want your quilting to show. I tend to quilt in a grey or white thread as those blend in well with just about anything except a really dark color and even gray blends in fairly well there. I usually quilt my log cabin in a diagonal straight line. It is easy to do because you can just go from corner of one strip to the corner of another so there is very little marking needed.
For free motion I find loops easier for me than meandering. I always seem to back myself into a corner meandering.
I generally start near the middle when I'm quilting but if you have your quilt basted together well there isn't a whole lot of shifting if you use a walking foot so if you don't start right in the middle it should be fine. Basting well is really the trick here. I glue baste but have also pin pasted before that. I like the glue baste because there isn't as much bulk and weight as with pin basting.