Under skilled worker category, while filling app for PR, point 20 says:
'Where do you intend to live in canada?
City, Province

So what should I write as I don't know anybody in canada?

You can write any specific place like Toronto, Ontario or Vancouver, B.C or Calgary, Alberta.
But you can enter Canada anywhere & work any place
Don't worry about that

Thanks.
Which is the international airport closet to usa and the city good for starting new life in Canada?

There are several airports close to the US border, depending on where you live in the US.
Toronto, Vancouver and Montreal probably are the best bets (in that order).
If you live on the US west coast, then Vancouver is probably easier.

Going back to your original question - I would be careful in what you state.
Don't just randomly name any city.

The reason is that in cases that are scrutinized or called for interview, you will be asked to give precise reasons on why you wish to settle in that city.
You will be asked to show evidence of your research and prove how you plan to settle in that city, apply for jobs, get a job, etc.

For example, if you are in the oil drilling business and you selected Niagara Falls, ON and you are asked to prove your settlement plans, you may have a challenge on your hands.

So, do your research and then fill it in.

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"Mah deah, there is much more money to be made in the destruction of civilization than in building it up."

-- Rhett Butler in "Gone with the Wind"

Ok Pratik sir. i'll follow your advice.