ocnbrze is correct. Read, read, read. I read the "ROOTing for Dummies" section 6 times over while scouring YouTube on how to root my phone and what to expect. If you just bought your Evo, I would suggest you take some time with it and learn its ins-and-outs before you root - the Evo, as a standalone phone is phenomenal - rooting just gives you more access to your phone.
When you realize that you truly are ready to root (and you've done your homework!), go for it! like is said, I used the "ROOTing for Dummies" and got it rooted on the first try. After you've rooted AND MADE A NANDROID BACKUP, sorry didn't mean to yell but nandroids are very important, you need to find out what type of ROM you want to flash; either a AOSP or a Sense ROM, there are pros and cons to both ROMs but I won't go into that. You can check them out here:
http://androidforums.com/evo-4g-all-things-root/284413-roms-kernels-download-page.html
When you've decided what ROM to use, you may want to flash a kernel to it, for Sense-based ROMs, netarchy-toastmod are the way to go and for AOSP, you can use Salvage-Zen, Ziggy or, my personal fav, Tiamat. But, once again, your going to have to do your fair share of research to find out what you really want/need.
And ultimately, that's the real fun about rooting, you get to experiment with different ROMs, kernels, themes, the CPU, widgets and homescreens - you get to personally customize YOUR phone to how YOU want it! I can't tell you how many times in the last couple of months I've come across Evo users and not one has had their layout the same, hell, even the rooted users I know tend to change their ROMs every weekend!
Have fun, learn more about your phone and make sure you do nandroid backups!