Franz Schubert's 'Allegro' D.154 is an early, energetic movement, possibly intended as part of a sonata or sonatina. It exhibits the youthful exuberance and melodic inventiveness characteristic of Schubert's early works.
Maintaining clarity in the Alberti bass accompaniment while singing the melodic line is a key technical challenge.
Allegro (D.154) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
Allegro (D.154) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.