Music for the triangle is written on the percussion staff, often with triangular shaped notes, but is sometimes written with regular music notes.  Triangles have an indefinite pitch, the notes will appear on the same line often as some of the other such instruments, such as bass drum and cymbal. The same note values apply for the triangle, i.e. a whole note, half note and quarter note, eighth notes, etc. Playing the triangle gets harder the shorter the notes are, so this is a really important skill to master. Graces notes are notes played in a specific rhythm before the first note in a musical measure. Playing grace notes on a triangle is easier if you suspend the triangle on a music stand with clips and play with two matched beaters.

Summary:
Learn to read triangle notes. Practice grace notes.