I'm a big fan of diagonal quilting. For me it is easier than stitch in the ditch. It's a good beginner pattern. As for thread, it all depends on how much you want the quilting to show. I tend to pick a color that blends as much as possible to hide my goofs. One good way to decide is to just lay different colored threads on the quilt. Pick the one that you like best.
Great first project. Happy quilting.