The thought is to use cotton thread with cotton fabrics. Poly thread can cut the fabric after a while.
I use white thread for the lighter fabrics though some people prefer a very light grey color, a dark color for dark fabrics, audition thread colors with your fabrics. I like to find one color that works throughout the whole quilt.
I use an 80/11 needle for piecing and sometimes go to a 90/12 when I sew on the binding.