Impulse is the devil

Did you ever have an experience where you acted on a sudden impluse to do something and regretted it later? I have one.  (<em>Because you're telling a story about something that happened to you, I put your verbs here in the past tense. You also need the "it" to refer to the previous action</em>.)

Once, I went shopping by myself. I saw a beautiful bag in the window of an expensive shop and I liked it very much. I went in and looked at it for a moment, but nobody come to greet me.
I tried it on when a sarcastic voice above me said:' the bag is very expensive, if you can not afford it, please don't ask to see it.'  (<em>above me instead of "up</em>")
To my surprise, I saw a middle-aged woman who was a large,unsmiling lady in a tight black dress. She came down from the second floor of the shop. I was very astonished at what she said. (<em>past tense!</em>)
'How do you know I can not afford it?' I said angrily.
The woman was busy cleaning her shop and did not pay attention to me at all.' So do you want to buy it?' she glanced at me scornfully and asked drily.
I regretted answering in the way I did, but I said:'Excuse my interrupting you. I hate to interrupt a woman when she is busy working, but would you mind wrapping it up for me?'
'Sure, I would be very happy to do that.' she answered with smile. Meanwhile, she quickly wrapped and handed it to me. 'One thousand yuan, please.' (<em>We know she's going to wrap it up for you by  now, so the future tense works best here.</em>)
'It is very expensive.' I said to myself. I paid the money reluctantlly and took it out of the shop. Even if I liked it, I did not have any clothes to match it.There is a saying is that impulse is the devil. And that day the devil provided me a very expensive lesson.

Your writing in English is pretty good! You seem to have a good hold on the past tense. There was only one spelling error- excellent!

It's not difficult to speak Chinese

Dear friends. Are you learning Chinese?  It's not difficult to speak Chinese.

It's pretty easy if you practice enough.

Don't forge the question mark! (;

My favorite singer is Declan Gailbraith.

He is really a great singer, and he showed his talent when he was only 8 years old. The song "tell me why" shook me,and when I heard "an angel",  I was really fascinated. His voice was so wonderful. There are many fans of him in China, they call themselves "December",and they all crazy about him. Declan was born in december of 1991, the same year and month as me. Although his voice has changed, it's not as sweet as it initially was,but it is still attractive. He had a concert in China in 2008,and his news is infrequent recently. I hope he can give more beautiful songs to us.

Okay! Not a bad entry! Here's a few things I corrected:

I (我）in English is always capitalized, as is the beginning of each new sentence.

Songs are mentioned in quotes in English, like this: "Tell me why". (We usually capitalize each new word in a song too, but this isn't always true.)  So,  <An angel> = "An angel"  ;-)

In English, the plural of a word ending in f  turns into "ves". Drop the f, and add the ves. (There will be irregular plurals with f, though, so be careful!)

The phrase is, "crazy about". Whether for a singer, sports or a TV show.

Also, "the same as" is a standard phrase, too.  Example: You speak English, the same as me!
Note: The phrase, "just like me" also works well here, but there will be nothing between like and me.

"it initially was" - This is past tense, so I had to change it from initial, and put a "was" to indicate the past.

"concert in China"- Instead of "China-concert" looks better in English. I know that in Chinese, you state the largest place first, then work your way down to the smallest place (ex. China, Zhejiang Province, Hangzou, 335 Beijing Road...). In English, it goes the opposite way- from small to big!

I hope this helped you out!