Today I'm going to see a doctor.

I told him that my back is sohas been itching a lot for some time.

To stop thatit, I tryied to put on some cream which I buythat I bought from a local drugstore.

When he heard my answer, he become a kind ofgot a little angry.

AndHe said to me, “wWhy does everyone try to be their own doctor?”

"If you gothave any health problems, you should come to see me immediately."

If that happen 10 years ago.

I'm not quite sure what you mean by this? If you leave a comment explaining further, I'm happy to help you figure out a better way to say it.

I will said to him “I am the customer,should have said, “Hey, I'm the one who's pay you the money, you better watching for this!”

I think by "you better watch" you mean "you'd better watch your mouth," but that phrase sounds too aggressive in this context.

But for now I just said, “I a'm sorry, no. I won't do that next time.”

Today I gowent to see a doctor.

I told him that my back ishas been so itching for some time.

To stop that I try to pute itching, I tried some cream which I buyought from a local drugstore.

When he heard my answer, he become a kind of angrygot a little upset.

"If you gothave any health problemissues, you should come to see me immediately."

If thatis had happened 10 years ago.

I willould have said to him “I a'm the customer,one who's pay you the money, you better watching for this!”

But for now I just said, “I am sorry, no next timeI won't do it again.”

Today I gowent to see a doctor.

I told him that my back is sohas been itching for some time.

To stop thait I try to putied some cream which I buyought from the local drugstore.

When he heard my answer, he become a kind of angry.

And said to me, “wWhy does everyone try to be their own doctor?”

I thinking joining the 2 previous sentences is most natural:
When he heard my answer, he become kind of angry and said to me, “Why does everyone try to be their own doctor?”

If you gothave any health problems, you should come to see me immediately.

If thatis happened 10 years ago.

I willould saidy to him, “I am the customer, who pay you the moneya paying customer, you better watchbe careful!”

Again, let's join the 2 sentences together:
If this happened 10 years ago I would say to him, “I am a paying customer, you better be careful!”

But nowtoday I just said “I am sorry, no next timeI won't do it again.”