understanding templates

I am new to all of this, so I am sure this is a very stupid question. I need to know what to look for in a Theme so I can choose wisely. Is there a tutorial somewhere that explains the differences in the themes….such as, how to tell if the theme is adsense ready, plugins that will work, if the theme can be edited, the best type of theme for a blog vs. a website? I have looked for a template with a picture of mountains or waterfalls in the mountains with no success, so do I choose a template that has an area at the top for my own pic, and if so how can I tell by looking at the templates. I haven’t found any info explaining this.