Hallo there

'Saya' is used in a formal situation/context, it is also used when you speak/write to  a person you respect or should respect ( including the eldery, people in authority and people consider in higher position).  As Indonesians consider the eldery, people in authority and people in higher position should be respected and should be spoken to in a formal way, therefore you should address yourself with 'saya'. 

'Aku' is used in an informal situation/context; between friends and equals.

I hope this answers your question.

Best wishes,

Sri Wilkinson