Mozart's Adagio in C minor (K. 356) is a brief but poignant keyboard piece, originally written for mechanical organ. It is noted for its elegant melody and expressive pathos.
Focus on creating a singing tone and conveying the underlying sadness through subtle dynamics.
Adagio (K.356) is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Adagio (K.356) lasts about 4 minutes.
Adagio (K.356) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.