Narrow the width of posts

I’m new to html and css so a simplistic answer would be appreciated.

Whenever I post a blog on my page it’s way too wide. If you’re not reading it in a browser that’s fully maximized, you have to use the scroll bar at the bottom to read it all. I’d like to be able to lessen/narrow the size of the actual posts themselves. I tried some html/css codes I found online but that just simply cut the post off at a certain point and did not simply reduce the width of the post itself.

What is the easiest/best way to narrow the width of the posts? I’m using Custom Community theme.