Johann Baptist Cramer's 'Adagio in D major' is a graceful and melodious piece, often used for pedagogical purposes due to its clear melodic structure.
Focus on a smooth and even tone production, paying close attention to the subtle dynamic shadings.
Adagio in D major is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
Adagio in D major is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.