"Calm down, Mr. Geller." I said.  
"I can help you, I have a trick about writing postcards"
Mr. Geller becames happly at once,then I bring brought him to my house, and gave him a book called 《Postcards》.I told him he just needed to copy the text of 《Postcards》and replace the names.That's it.
Guees what? Mr. Geller wrote fifty postcards that afternoon. 
I saw Mr. Geller at dinner time again. He looks very well.
I went toward him, then Mr Geller say said hello to me, and gave me a 100 pounds for my help and gave me another 100 pounds wanted to buy my 《Proscards》. I readily agreed.
After a few days, Mr. Geller left. He said thanks to me again before he left.
Now I'm looking at this postcard on my hand, and feel a little touched.
"What a kind man!" I thought.

A postcard from Mr Geller.

[Very nice story. Good job! I'm glad you're learning English.]

A postcard from Mr Geller.I'm a waiter, and I work in a restaurant in Italy. Note > A comma is needed to separate the 2 independent sentences, and it's placed before a conjunction ( and , but, etc. ) Another option : I an a waiter who works in a restaurant in Italy.
I got a postcard this morning, the signature is Mr Geller.X > I received a postcard from Mr. Geller this morning./ I received a postcard this morning that has Mr. Geller's signature on it.

"Mr Geller finally learned how to write postcard or still copy." I thought. > " Mr. Geller has finally learned how to write something on a postcard, or is he still copying them " I thought.

There are a lot of people lived in our restaurant every year on holiday, and Mr Geller is one of them. > There are a lot of people who like to frequent our restaurant every year whenever they come here on holiday , and Mr. Geller is one of them.
He is a very kind man with a black glasses, looks like a professor. so I'm very glad serve for him. > He is a very kind man who is always wearing a pair of dark glasses, which makes him looking much like a professor. I am glad to serve him.
One day, I saw Mr.Geller, so I say hello to him, and he say hello to me too. > One day, I saw Mr.Geller, so I said hello to him, and he said hello to me in return.
Mr. Gellers seems in a little trouble, because his face don't look well( sick?)> He seemed a little troubled, because he did not look like his usual self.
"Mr. Geller, What's up?( informal lingo among friends) You don't look well." I said politely.> " Mr.Geller, is there anything wrong? You seemed troubled," I asked him politely.
"Oh! Yes! In fect I'm in trouble." Mr Geller whisper sadly. > "Oh, yes! I do have some problem in my mind  as a matter of fact," Mr Geller replied softly in a sad voice.
"What is it? Maybe I can help you. " I said. > " What is it? Maybe I can be of some help," I told him.
Mr Geller seems a little embarrassed.> Hearing that, Mr. Geller seemed a bit embarrassed at first.
Then he muttered:"oh...Is that...I need to write some postcard, but I can't do it." > Then he muttered: " Oh, ...the thing is... I have to send some postcards, but I do not know what to write about".
He saw me puzzled. Then he continued > He saw the puzzled look on my face, and then, with some disappointment in his voice, he added: "I just can't do it, you know? There is always something that is just too difficult for some people. I bought thirty seven postcards, and I had stayed home all day trying to write something on them, but I still did not even finish with any of them".   Mr Geller are little excited.
"Calm down, Mr Geller," I said, "I can help you, I know have a way that can help you overcome that problem".  write what to write about write postcard"
When he heard that, Mr Geller became happy immediately. at once, then Later, I took him to my house and bring him to my house, gave him a book called" Postcard". 《Postcards》I told him that he could copy any text from the book but just had to make sure that he replaced any name with his own .That was all there was to it ./ I was as simple as that.
Guees what? ( This is not a question. Its just a remark) Mr Geller wrote fifty postcards a afternoon. > Guess what! Mr. Geller wrote 50 postcards that very afternoon!

I am running out of space for correction. Keep it shorter the next time I  stop here.

A postcard from Mr Geller.