How to make a teaser

I want to write a very clean code and don’t want to put any code in the blog posts themselves as CSS can do most of the things I need, but I’m not familiar with other codes to well currently.

I want to make an automatic teaser that shows the first image and the first paragraph (maybe 100 words if you can’t do the paragraph automatically). I also want to put in a “read more” link at the end of it.

Anyone know how to do this?