how to post on specific page ?

I’m sorry, I don’t understand what you mean.

Pages are normally used for static content, so about, services, contact, etc… Content that generally doesn’t change too often.

Posts on the other hand are used for content that get updated. Historically, it was a blog, but it could be news, portfolio, thoughts, press releases… etc.. Posts can be assigned to categories.
So to answer your question, you could write blog posts and then assign them to a category.

So have to create different categories .
and while posting choose different appropriate categories ?

Yes, if you want to have different categories, otherwise, you can just have a blog and display all of your posts.

You should think as categories as ways to help your users. Think of it as a menu. Breakfast, lunch, dinner. Your blog can be the same, but perhaps have a few more categories.