I often to see this kind of question from people who speaks English as their mother tongue, and they tend to be confused as they don't have variation in calling/referring themselves.

In short explanation, Saya is the formal way to call yourself, and Aku is also a way to call yourself used in everyday conversation (as a mere option), and it's informal. Also there's other way to call yourself used by young people that I will tell you later.

FYI or As you may know, Indonesia is a country that formed from many tribes where every tribes have their own unique language. Indonesian language is an official language which is made for the various people across the country to be able to communicate with each other with only one language. And the formal and the informal language is necessary as a purpose of fulfilling on any conversations and adding a nuance to the language itself.

If you feel it's hard to use, you can easily pick Saya to refer to yourself. Saya is not only formal, but it's universal and can be accepted by everyone.

The same case as Saya and Aku (English; "I") which is the formal and informal way to referring yourself, there's Anda and Kamu (English; "You (singular)") which is the formal and informal way to referring the people you're talking to. I'll give you a little example of the variant below;

I = Saya (formal and universal), Aku (informal), Gua (slang)

You = Anda (formal and polite), Kamu (informal and universal), Elu (slang)

They/them = Mereka ; You (plural) = Kalian

The variant above isn't made to make this language difficult, but it is a necessity to make Indonesian language as a language that can be used in every situation for every people, adding a variant, and give a nuance to the language itself.

p.s In another case, you still can find this kind of words that changed even though it does mean the same literally, but they always be accepted differently for every people you talk to. Bahasa Indonesia can be said as a language that's still developing.

Yes, I was quite confused, especially in these subjects. Also, prepositions like to with are constantly changing. Or the order in which a sentence is formed is not always the same. Can you tell me how to form a sentence? Finally, it just says dia for he and she. How should I know if it's he or she in this case? Her, him, us, these words also seem to be constantly changing. This is the only part that I really have difficulty with this language. It seems like it changes a lot from condition to condition from moment to moment. I can't find a pattern. This allows me to learn words but not be able to form sentences. What do you recommend ?