Mary Ann,
When I make clothes, I wash my fabrics first, so that the finished garments don't shrink. Use a poly thread, cotton thread doesn't have the give you need for a knit fabric.
When working with t-shirt knits make sure that the fabric stretch is running horizontally(around the body) or your garment won't fit. The old Stretch n' Sew patterns were wonderful, but I don't think they come in plus sizes. Simplicity has some easy patterns.
Sharon in Texas