Play notes that fit within the key of the song. (Ska players rarely deviate from the key.) Stick to playful melodies around major scales. For example, if the song is in the key of C, play notes in the C major scale.
Play syncopated notes that deviate from the bass line. The bass is usually on the upbeat and the guitar is usually on the backbeat. To play syncopated notes, learn to string melodies on the backbeat. The backbeat is the "and" of the beat (1 and 2 and 3 and 4 and).
Play bright and tight. Concentrate on keeping your notes short and tied to the backbeat.