You cant beat just rolling up your sleeves an diving in.
Maybe start with simple things like tweaking the CSS a bit,
and then start playing with the theme php.
Find a blog you like the look of for design inspiration, then work towards something like that. Usually its easier if you start with a theme that has most of the features you want in the first place.
There's tons of info out there for the Googling, but you'll only know what you need to Google when you start playing ;)
If doing that on your live blog is a bit scarey I'd recommend you create a local copy of your site on your computer.
I use a program called Serverpress ( http://serverpress.com/ )
It makes it super easy to have WordPress running right on your desktop so you can play to your hearts content without breaking your live site.
Beware though, once you start tweaking your theme, you might get addicted and end up deciding to make your own (like I did). ;)