Unless you already have a little icon you'd like to use, you'll probably need to choose a simple, clean image from a site like IconArchive.
Here's one of my posts with an unordered list, using cute bullets.
Upload your chosen image to an image hosting site like Photobucket, and make a note of the image's url.
Next, remember to always backup your template first before making changes!
1. Go to your dashboard Layout---Edit HTML
2. Find these lines of code in your template file
5. Save your template if you're happy with the results.
Leave a comment and let me know how you go!