I don't know how much you do or don't know about snowboarding, but here are some recomendations. If you already know all this then go ahead and skip to the end) DO NOT BUY STEP-IN (unless they are the ones that the back folds down on). Step ins are cool and seem convient...until snow gets packed in to them and your foot slips out going down the hill. Also make sure the heel cup is metal not plastic.
Step-ins are okay if you like them. they are easier to learn in since it is easier to get your foot in and out. You should never have the problem of you foot slipping out going downhill if yu are properly atatched before you head that way. It takes a few seconds to scrape the snow off you boots before you get in but you also have to do this with strap-ins as well and they have a longelearning time to get used to. the only thing metal you want on your board are the screws and the edges, everything else should be plastic or wood.
There are two types of boards sidewall and capped construction. As a beginner you can't much tell the difference, I now prefer sidewall.
The size should be between her chin and nose (if she is young go towards the nose so it will last longer).
The length should be her chin or below if she is a beginner and to her mouth if she is intermediate and to her her nose if she is good. Do not buy her something she will grow into! It is not good for them to learn on something like that.
If she is a beginner make sure to get her a nice helmet. I took some spills when I first started that had i not had a helmet I would have been in trouble.
GET A HELMET NO MATTER! It is not just in case she falls, it helps if someone else runs into her as well. Some mountains also require this.
My last recomendation is to buy her a stomp pad, it just makes getting off the lift easier, some boards even come with them.
Most mountains REQUIRE a stomp pad so this is a must if it is not already on the board.
www.sierratradingpost.com (check out fatwallet.com for free shipping offers)
www.backcountryoutlet.com
and check out your local sporting goods store sometimes their sales are amazing!