My colleague has left my company

Taday Today My colleague has left my company. She is my good friend. She taught me a lot of things.We worked overtime,have had dinner and danced together.Now,she left the company,i am sorry.I wish her the best of luck

You could also say:  "'She was my good friend."  Although she is still your friend, if you will not be seeing her regularly again, you could use "was."

Speaking of the things the two of you would do together, you could use the past tense but you could also use "would." "Would" can indicate repitition in the past.  "We would often work overtime."

It's a minor thing, but in your first sentence I would use the simple past "left," rather than "has left."   Both are acceptable but we often try not to repeat words or expressions in close proximity.  Since "has left " was used in the previous sentence (the title) it would sound better to my ears to change the wording a bit.

你的英文很不错。用地道的英语我会这样写。

Today my colleague left the company.

My colleague quit her job today.  She was a good friend of mine.  She taught me a lot.  We would work overtime together, eat dinner together and go to dances together.  Now she's gone and I miss her.  I wish her the best of luck.

"A lot" is commonly used for "a lot of sth."

I used "a" good friend of mine"to indicate that she was only one of your friends."

I'm not sure what you meant by "danced together."  "Danced together" could mean that the two of you danced with each other.  "Go to dances together" means that the two of you went to dances and danced with others.