Finding good quality, good fit basics in a plus size can be tricky if you don't want to pay a bucket load. One great purchase that I have bought and repurchased more than once is basic full length leggings from Autograph they are $29.99. They are great for night/day time with a long top or going to the gym. They are opaque and the material is breathable, they also keep their fit well. I personally have 5 pairs... they are just so comfortable!
A great fitting tee is hard to find. You want one that is long enough but not to long, a decent size sleeve, a flattering neckline and I personally do not like tees that slack at the back of my neck. Here are a few that I think are worth a try.