I second using ABRSM [1] material, particularly for classical music. It's graded and structured with workbooks (and even online theory exams if you want). ABRSM

I second using ABRSM [1] material, particularly for classical music. It's graded and structured with workbooks (and even online theory exams if you want). ABRSM material and exams are for anyone and recognized for university/conservatoire entry at least in the UK. They start really gently at Grade I. [1] https://gb.abrsm.org/en/our-exams/online-theory/ https://gb.abrsm.org/en/our-exams/online-theory/