OK, I'm going to be blunt.
You say you're sort of a newbie, but you didn't follow the tut's advice and use the pen tool to make your selection. Your selection is awful, total newbie at renders, not mincing words here.
This tutorial is very long, interesting, but very involved and very likely way beyond your skill level. Certainly with the pen tool it is. You so need to learn the basics of PS and find tutorials that challenge you but don't stump you for the time being.
We all have to accept our limitations at some point. Well, "we" whoever "we" applies to, do not of course have to accept anything more than we wish to accept, lol, but that may truly hinder creating good results and learning new skills.
I do admire your desire to push yourself, but like you said, you're struggling because this is beyond your current abilities. I do hope my blunt criticism doesn't hurt your feelings, but I do think
there are plenty of tutorials out there that will give you lighting effects like this. In fact, we just finished a Forum Challenge that included lots of links to whole sets of tuts. Just be sure you pick one that's a bit shorter and a little bit simpler. Here is a
link to Challenge 11. I know you will find a good one to use. You can also download brushes for lots of sparkle effects (I suggest going to deviantarts.com) so you can customize your own touches to the girl. She's very pretty btw.
If you look up some tutorials on creating renders, you will find lots of advice, like using refine edge or mask. If you like, just ask, and one of us, myself included, can outline some steps for you to use. Also, if you search making selections here on the forum, you should find something. Sometimes you have to use several different search terms or you'll get a jumble of results.
I wish you lots of luck with this. Is this a gift or practice for your own Photoshop growth?
We'll help you. I promise!