Aku and saya are the two most using word for referring singular first person like ‘I’ in English. Aku is informal and saya is formal. For some people, aku is a little bit childish and using by mainly children and girl who wants to sound cute or using in informal communication. In some area such as Javanese language realm, aku is normal because is akin and similar with Javanese for informal singular first person.
Do you know that Indonesian also have another words for singular first person pronoun, such as beta (using by Kupang Malay, also Maluku Malay and Papuan Malay speakers), hamba (using by people who have lower status or rank, now is obsolete and we find only in stories), awak (using by Sumatran Malay speakers). Please take a note that awak in Malaysian Malay is for referring informal singular second person or kamu in Indonesian.