How to find a good ninche?

Do you mean a niche?

The best way of finding a niche with a hungry market is actually looking around you and writing things down. Do this for a week. Then use a keyword tool and start filling out all the things you found one at a time. Save any keyword you come across that has much less competition than the rest but more searches than the others. If you're planning on becoming an affiliate marketeer then the next step would be looking for the most suitable product as well as looking for the problems people are facing.

There are great articles on the internet about affiliate marketing on the internet, from niche inspections to list building...

Find a general topic, then zoom into the topic layer by layer asking yourself what are the sub-categories of these topics that you have selected. This way, you are able to zoom into the niche of any general topic. But, always make sure that you love what you write and know what you write about your selected niches.
if you would like to know more about content publishing and writing on niches, you can go to my Profile page and click on My Website to find out more. I have written a website just for such things.
Good luck.

cats are a good choice. a lot of people have cats as pets and know nothing about cat caring you will get a lot of clicks. but you want get much of expensive advertisements cat food dont sell as dog food does

A very difficult task. First part is easy what people are looking for. But the second part is too tough.
What men look for is covered up instantly. So you can think what people will look for after 1 year and then plan for that. if you find something, let us know:)
Have a great day.