Excerpt on home page instead of full content – how do I change it?

I am using: WordPress 4.1.1 running Twenty Eleven theme.

I have used the search function for the last hour but all the answers seem to be very old and don’t work.

I want all of my postings to just show the first 10-15 lines and then the reader can click on the page to read the entire post.

What specific pages do I need to change and what changes need to be made?

It looks like I need to change “content” to “excerpt” but I’m not sure which page I need to make those changes.

https://davidbaileyblog.com/page/3/

Thanks in advance