Category & Tag Search with Title & Thumbnail only

I’m beginning to have too many posts per category so it takes up too many pages to have the full content when you click on a category. I would like to change my functions.php to only include the post title and maybe a thumbnail. I’m not sure how to do this and the other tutorials I have found in my search don’t work.

Any help is extremely appreciated! My site is http://joshuatrent.com The cooking section is the primary reason I need this change. Thanks.