Jailbreaking the phone is the easy part. It's the customization that has a learning curve to it. Basically, once you jailbreak your phone, it will look exactly like the phone you have now, with the exception of the Cydia app store icon and access to same.
My suggestion is this: jailbreak it first. Browse the forums for a particular look or theme you are looking for, and then PM the user or post up for the information on how to achieve it. That's how I did it. I saw someone using the HTC clock theme on their home page, and asked how to do it. There's lots of tutorials on Youtube also about themes.
Some of the tweaks can be pretty involved, like going into the filesystem and altering lines of code, etc. If you have never used a Linux setup or are not familiar with terminal commands, blah blah blah then just stick with the basic themes and some of the more common apps.
I had a Droid before my 4S, and I always wanted to root it, but was intimidated to do it. The jailbreak process is so easy, I'd do it again and again. I love having a jailbroken phone now. And if you screw it up, just connect to your computer, fire up iTunes, and restore your phone. Back to normal!
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