About a car

A friend of mine bought a car. He was glad to get it because his old car's air conditioner had broken.
He said "My new car's fuel efficiency is good compared with old car." So I asked him "How far does it run per litre?"　He answered me "It runs about 12 kilo metres per litre. Old one runs about 6 or 7 kilo metres per litre."
He often drives his car so it would help him to save his money.

By the way, I have a car licence. But I can hardly drive because when I got my car licence I lived in the city,so I didn't have a chance to drive.
I start to practice to drive my car these days. But I can only go straight or turn left! I'll do my best!

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Corrections:

 A friend of mine bought a car. He was glad to get it because his old cars air conditioner had broken. 

He said "My new car's fuel efficiency is good compared with the old car." So I asked him; "How far does it run per litre?"　He answered me; "It runs about 12 kilometres per litre. The old one runs about 6 or 7 kilometres per litre." 
He often drives his car so it would help him to save his money. 

By the way; I have a car licence. But I can hardly drive though because when I got my car licence I lived in the city,so I didn't have a chance to drive. 
I start to practice to drive I've started to practice driving my car these days, . Bbut I can only go straight or turn left! I'll do my best!

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Explanations:

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A couple of things: when using speech you should use ; before it. Example:
He said to me; "What do you do for a living?"

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Kilometres (or kilometers depending on whether it is British or American English). Both are one word though.

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In the situation here:
My new car's fuel efficiency is good compared with to the old car." 

You use "the". So "compared with the old car", because there was only one, and it was a specific car, not a "general" car.

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 save his money. 

You can just say "help him to save money". You don't need to say "his" because you have already say "him". The only time you need to say his in addition, is if you were talking about two or more people (or if there was more money than "his" own). Example:

I want to help him save money.
She said to let her pay for it, and for him to save his money.

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By the way; I have a car licence. 

When using sentences like this; you should use a semi-colon. You "can" get away with using a comma, but technically it should be ";".

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I have a car licence. But I can hardly drive 

You never start a sentence with "but" because it's a "joining" word, like "and". There are exceptions, but they are kind of complex and you shouldn't worry about them until later.

So:

I have a car license, but I can hardly drive

or

I have a car license. I can hardly drive though

One or the other, but you don't use "but" after a full stop. :)

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I start to practice to drive 

should be:

 I've started to practice driving

 When using "practice" it is usually either:

I start practicing (tomorrow/future).

or

I've started to practice (today/past).

The differences are that if you say "start" it actually implies the future unless otherwise mentioned, because it hasn't "started" yet.

If you say "started" it means it has already begun. 

(An example of when "start" is used for past tense):

"When did he start doing that?" - Past tense - already implied with "did", so you don't need to say "started".

It's complicated, but it just takes practice. Even I think it's silly, but that is fairly advanced English.

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I hope it's not too advanced for you. They are only minor mistakes. :)