Contrary to JCarey, I won't recommend the D minor Fantasy nor the A minor rondo. Both are highly mature work and requires a lot of finesse and thorough understanding of Mozart's idioms. I have not heard many good rendition of the D minor Fantasy played by students even though it is often played. I would second what j-menz recommends: run through a couple of the sonatas; some of the sonatas have easier movements: such as K. 279, K. 280 (2nd), K. 282 (2nd), K. 283 (which is my favorite for starting my students on Mozart sonatas, since I dislike K. 545), etc.
Have fun.