Great night

I had a fun time this afternoon, so I am very happy. I sung in a karaoke bar with my friends. I can't sing very well. Some of my friends can sing very well, because they are members of a Chorus.

Good job! 很好! I changed 'Happy night' to 'Great night' because usually we don't say Happy night, even though grammatically it makes sense. I changed 'have' to 'had' because you had already sung in the karaoke bar. So you needed the past form of 'have', which is 'had'.

Verb: Have

Past tense: had

Present tense: have

Future tense: will have

I added the word 'so' to connect "I had a fun time this afternoon" with "I am very happy." If you don't have the word 'so' or another connection word or phrase, then you have an improper sentence due to the comma. If you want it to be two sentences, there should be a period after the word 'afternoon':

"I had a fun time this afternoon. I am very happy."

The word 'a' was needed before the word Karaoke. Just for the sake of learning: The words 'a' and 'an' are basically the same thing, however, 'a' is used for words that start with a consonant. 

Ex: a Karaoke bar, a house, a car, a phone.

However, there are some exceptions to this rule that you should be aware of. 'A' is also used with vowels that have a consonant sound:

Ex: a unique...

Because the 'u' in 'unique' soungs like 'yu' we use an 'a'.

Now 'an' is used for words that start with a vowel.

Ex: an earing, an open book, an apple

There are some exceptions to this rule as well. When we have a silent consonant as the first letter of a word we use 'an'.

Ex: It is an honor.

Since the 'h' is not spoken, 'honor' sounds like 'onor' which is why we need 'an'.

The word 'because' is used just as I used 'so', to connect the two sentences. 'Because' 是因为.

I hope this helped you! I wish you the best!