Please check this joke for idiomatic sentences:

Customer: (To the milkman)
I don’t know, why, yesterday the milk was sour?
Milkman: Madam, may the cow had eaten a few lemons the previous day.
That may be the reason for the sour taste of the milk.

Customer (to milkman):
The miolk was sour yesterday. Can you explain why?
Maybe the cow ate a few lemons the day before, Madam.

(Note ‘maybe’ not ‘may’. ‘The day before’ is idiomatic. The meaning that it was the day before the incident being discussed occurred is assumed.)

Please check whether all sentences are idiomatic:

First Friend: Please give me just one rupee coin.
I want to go back home by bus, but I have no change.
Second Friend: I don’t have any change(=coins), but I have only twenty rupees note.
First Friend: That is a great help.
Give me that twenty rupees note, I will go home by auto.

Looks good to me.

There was a Bowery Boys movie where a cop arrests Slip Mahoney.
Slip says, “Hey, I’m entitled to one phone call”
The cop says, there’s a pay phone right there, use it and he points to a phone on the wall.
Slip says, “Got a nickle?” (a nickle is five cents)
The cop digs in his pocket, looks at his pakm. “Sorry, all I have is a dime” (a dime is ten cents)
"That’s OK,’ says Slip. “I can make change” and Slip plucks the dime out of the cop’s hand, leaving a nickel.
I don’t know exactly when that movie was made; probably the 1930s. Pay phones were 10c in the 1950s.

I think “twenty rupees note” should be “twenty rupee note”.
Also, “I have only a twenty rupee note”

To make it sound more idiomatic I would make these changes:

“Please give me just one rupee coin.” -> “Can you spare a rupee coin”?
“I have no change.” -> “I’m out of change”.
“I will go home by auto.” -> “I’ll hail a cab” or “I’ll take a taxi”.

Thanks for help.
Please extend your further help on the following joke.

Guest: (To the little girl)
Why baby, was your dog barking at me while I was taking my meal?
Little girl: Uncle, he’s angry because you were taking meals in his plate.

1st person: Please can you lend me one rupee? I don’t have change for the bus.
2nd person: Sorry, I only have a twenty rupee note.
1st person: That will do. Give me the twenty rupees and I’ll go home by taxi!

Guest: Why was your dog barking at me while I was eating?
Girl: He’s angry because you were using his plate.

Not a native English speaker, then.

I add on one more.
I am always counting your helps.

Patient going for operation: Doctor, I am too much afraid of death.
Doctor: Don’t be afraid. I am here only.
Patient going for operation: Doctor, I know you will be here.
Will I be here, Doctor?

This may be another problematic one, as far as translating into English is concerned. Is the joke that “be here”, which the doctor meant as “be present”, can be misunderstood as “be alive”?

I also don’t understand the purpose of “only” in “I am here only”. (Afterthought: I guess you mean “I’m just here” or “I’m right here”.)

I have modified as per your instruction.
Please check: is it OK now or not?

Patient going for operation: Doctor, I am too much afraid of death.
Doctor: Don’t be afraid. I’m right here.
Patient going for operation: Doctor, I know you will be here.
Will I be here, Doctor?

Um, my post was not an instruction but a question.

Patient (before operation): Doctor, I’m really afraid of dying.
Doctor: There’s no need to be afraid. I’ll be here until it’s all over.
Patient: I know you’ll still be here. The question is, will I?

Thanks, Beeesneees & Dozy,
I am throwing one more. Please make it more effective joke.

Friend-A: Do you know what happened?
A stone thrown by your son fell upon my foot.
Friend-B: If someone fell upon your feet, just forgive.
Don’t mind it.

I can’t turn that into an effective joke. It doesn’t appear to have any humour in it. I can correct it so that it sounds natural in English, but it has no punchline as a joke.

A: What happened?
B: Your son threw a stone and it landed on my foot.
A: Please forgive him.

It could be you translated an idiom into English that has a double meaning in your language (and hence brings a smile) but not in English.
You may hit a wall while translating some idiomatic jokes into English.

There is an expression “fall at someone’s feet”, also “throw oneself at someone’s feet”, meaning to humble oneself towards someone (e.g. worshipping, being subservient, appealing for mercy, etc.). It may have been the intention to somehow play on this expression, but in the joke it is the stone that falls on someone’s feet, which doesn’t make sense with this expression.

Please help me to make the following joke idiomatically.

Teacher: My dear boys, you can ask me your doubts at any time without shy.
I will definitely help you and clear your doubts.
A naughty boy: Then that is a great help to us, sir.
I don’t think you will mind if I ask you at the examination hall.

Teacher: My dear boys, don’t be shy about asking me questions about your work at any time. I’ll always help you.
Boy: That’s great, Sir! I’ll ask you the answers when I’m taking the exam.

Thanks to Beeesneees,
I am adding one more. I want help from you all.

Police: Your bicycle has no brake, no light, no bell!
Come to the police-station to pay fine.
Person involved: Excuse me this time, sir.
Since it is night, I didn’t check all before
steeling this bicycle.