
Generally, you should use alternating 'i' and 'm' fingering. The score will often start with a suggested fingering in the first few measures and it's assumed you should continue with that until some other fingering is suggested in the score. Certain sections may need to be treated in a special way so you will see 'i', 'm', and 'a' symbols above the staff indicating what to use. 'p' symbols may appear below the staff where you should use the thumb in situations where you might not normally do so.
Hope this helps.
Best regards,
Bob G.