How to categorize posts in blog

I am trying to categorize the posts that are being made using the blog app . Below is what I'm trying to achieve

1. The user who writes the blog needs to choose the category of the post hes about to make . - How do i fetch this information ?
2. Use the same as a filter to display the posts under that category in a particular part of the page . - I understood this using ( perch_blog_custom ) and passing the filter parameters .

I read through the documentation of perch_blog_categories & category . Unfortunately they only show how to output the data .

1 . How do i take the input from user . 2 . How do i create categories ?