I have compiled a list of really good resources that walk you through how to do Thesis theme editing and customize it your liking.
Thesis Theme Editing:
Blogussion - 4 Great Ways I Customized my Thesis Theme - I love the Author at bottom of post tip!
SugarRae - Thesis Theme Custom 404 Page - No more "Gnarly, dude!"
DIYthemes - Hide "comments on the entry are closed" Message - Very helpful if you do not allow commenting on your blog or website.
ThesisThemeHQ - Making Changes to the Thesis Navigation Menu - Quick and easy but makes a dramatic design difference as you will see.
DIYthemes - Modify the "Tagged as:" Text - Something almost nobody does, but you should be doing!!
Thesis Theme Hooks How-To:
What you first need: Thesis OpenHook Plugin
DIYthemes - Thesis Hooks Reference List - Well compiled reference list for thesis hooks
I'll be updating this more soon!