Nice job! There are just a few awkward sentences and mistakes. I am impressed that you are using quite a lot of colloquial words. Since Italki is big on learning by talking rather than writing alone, I thought it may be fun for you to learn how to talk like a native would recount it in normal daily conversation;

Mau kemana?

Di pinggir jalan ada orang* indonesia duduk. Dia lihat ada* orang lain yang lagi/sedang* jalan, dia tanya "Mau kemana?"  Orang itu mengatakan/bilang* dia mau jalan ke pantai. Orang yang duduk terus/lalu* memberitahukan/bilang* "Salah* arah pak/bu jika anda mau pergi ke pantai!" Orang yang jalan terus/kemudian tanya "Bagaimana* pak?" Orang yang lagi duduk terus menjelaskan/bilang bahwa pantai itu letaknya di sebelah kiri, bukan di sebelah kanan. Orang yang jalan balik* arah dan bilang* (mengucapkan) terima kasih

*se in this case is optional in colloquial language. 

*ada = there is

* lagi or sedang is often used to describe the person is walking (the "ing" form)

*bilang =colloquial for mengatakan 

*terus = colloquial for lalu/kemudian

* Salah arah (=wrong direction) is more appropriate then saying Tidak! You can't say Tidak out of the blue. It's awkward to say NO unless there is a preceding question.

*bagaimana is better than mengapa in this case

*balik arah is colloquial for kembali arah and less awkward than berputar