Limit length of a wordpress post

My blog https://windowsvj.com/wpblog/ has a strange way of presenting the blog posts. It displays complete (full length) of blog posts.

What i want is to post an excerpt or a teaser of say 100 characters or 2-3 images and then have a read more link that makes user go to next page.

However i am not bale to modify the code/ find any plugin which solves my need.

I need help regarding the same. Thanks.