What can I use instead of 'Anda'?
I'm writing a letter to a penpal for my class. We were told not to use 'anda' as it is too formal for this situation. What can we use instead?

I'll give some examples of sentences to make this more clear:
Terima kasih atas surat Anda. Ya, saya mau menulis kepada Anda.
Could I use?: Terima kasih atas surat kamu. Ya, saya mau menulis kepada kamu.

Bolehkah saya bertanya?

Di mana alamat Anda? Kapan hari ulang tahun Anda? Berapa umur Anda?
Can I just put a person's name? Di mana alamat Olivia? Kapan hari ulang tahun Olivia? Berapa umur Olivia?

Bagaimana dengan keluarga Anda?
Bagaimana dengan keluarga kamu?

If anyone could show me how to write these sentences informally, without using Anda, I would really appreciate it!

Thank you! Terima Kasih

Question

What can I use instead of 'Anda'?
I'm writing a letter to a penpal for my class. We were told not to use 'anda' as it is too formal for this situation. What can we use instead?

I'll give some examples of sentences to make this more clear:
Terima kasih atas surat Anda. Ya, saya mau menulis kepada Anda. 
Could I use?:  Terima kasih atas surat kamu. Ya, saya mau menulis kepada kamu.

Bolehkah saya bertanya?

Di mana alamat Anda?  Kapan hari ulang tahun Anda? Berapa umur Anda?
Can I just put a person's name? Di mana alamat Olivia?  Kapan hari ulang tahun Olivia? Berapa umur Olivia?

Bagaimana dengan keluarga Anda?
Bagaimana dengan keluarga kamu?

If anyone could show me how to write these sentences informally, without using Anda, I would really appreciate it!

Thank you! Terima Kasih

Answer

Instead of anda, it might be better to use kamu or only mu (shorten version for kamu).
If it's really informal, you could use lu which is Indonesian slang for kamu or anda.

E.g. dimana alamat anda? --> dimana alamat kamu? OR dimana alamatmu?

Usually I prefer to use mu than kamu
Kapan hari ulang tahunmu? OR kapan kamu berulang tahun?
Berapa umurmu?
Bagaimana dengan keluargamu? --> this is a weird sentence. I'm not sure if you want to ask their conditions or you want to know more about his/her family or something else.

I hardly use someone's name in a sentence, except for the opening sentence / greetings.
E.g. Hi Olivia. Apa kabarmu?

Answer

indonesia teen usually say 'anda' with 'kamu' or 'loe'.
I'm(saya) with 'gue' or 'aku'
How(Bagaimana) with 'gimana'

tips from me, dont be shy to ask or kidding with her. 
for example you can ask about her life style, school life, her community friends, be fun friends..

'Sekolah dimana?'
'Biasanya kalo habis pulang sekolah ngapain?'

I hope this help :) (Saya harap ini membantu)

Answer

wow your indonesian is very stiff???? but i can help you~

if you want to use informal for 'anda' it's 'Kamu'.

if you want i can change your text into informal so that she can understand easily^^