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

Di pinggir jalan ada se orangseorang Indonesia sedang duduk.

1. Indonesian people uses the phrase "di pinggir jalan" for explaining an event that occurs on roadside.

2. "Seorang Indonesia" is phrase for "an Indonesian", it is necessary to put "se-" in Indonesian written language. But then, in spoken language it is optional.

3. "Someone are sitting." = "Seseorang sedang duduk." The word "sedang" is like "-ing" in English present continuos tense.

Dia melihat seseorang yang lainsedang berjalan-jalan, lalu dia mintabertanya, "Mau ke mana?".

1. You can just use "seseorang" (someone) instead "seseorang yang lain" (another person).

2. "Asks" (asking for answer) in Indonesian is "bertanya".  Whereas the word "minta" is for asking for something.

1. The word "pun" here is for emphasizes that the someone's here are said something.

2. The word "bahwa" here is for declares what has been someone's said.

3. The word "hendak/ingin" is formal word for "want". Whereas "mau" is an informal word. They are said "I want to go to the beach.", didn't they?

Seorang Indonesia yang sedang duduk itu memberitahukanmengatakan, "Tidak! Anda tidak pergi ke arah pantai!".

1. In Indonesian, it is commonly to use "Seorang Indonesia yang sedang duduk itu" (That Indonesian who are sitting) instead "Seorang Indonesia yang sedang duduk" (The Indonesian who are sitting).

2. The word "mengatakan" is better here since this sentence is in direct form.

3. "Arah"="way". Oh no! You're at the wrong way to go to the beach! :O

Seseorang yang berjalan-jalan itu bertanya, "Mengapa, Pak?". Seseorang yang sedang duduk itu menjelaskan bahwa pantai ada di kiri, tidak ada bukan di kanan. Seseorang yang berjalan-jalan itu pun berbalik arah dan ...

You better try not to make the sentences directly from dictionary word, you need to adjust some phrases to make it correctly in Indonesian. You can compare with my version

Di jalan ada orang  Indonesia yang sedang duduk. Dia melihat seseorang sedang berjalan dan kemudian bertanya "Mau ke mana?". Orang yang sedang berjalan tersebut mengatakan ia akan pergi ke pantai. Orang Indonesia kemudian berkata "Tidak! Anda tidak akan pergi ke pantai". Orang yang sedang berjalan kemudian bertanya "Kenapa Pak?". Orang Indonesia menjelaskan bahwa pantai terletak di arah sebelah kiri dan bukan di arah sebelah kanan. Orang yang sedang berjalan kemudian berputar dan mengucapkan terima kasih.

There is diferrence how to express some phrases in Indonesia than in English, altough both have same purpose. For example, at the situation, you know that the person walking on the wrong direction. In English you may you said "No, You won't go to the beach" as literarily mean as "Tidak, Anda tidak akan pergi ke pantai". I think this phrases tends to its object the walker never get if he goes to that direction.

In Indonesian phrases we stress on the person act by walking to the wrong direction. So we should said "Anda berjalan di arah yang salah, karena pantai terletak di arah sebelah kiri."

Wish helpful, Have a good time! :D

Mau ke mana?

It's better to use 'Bukan' instead ' Tidak', these two words can be translated as NO in English but 'Bukan' is better to express 'not this but that . . . '

I'm sorry my English not so good