Can someone help me build a MTG deck?

Decks are typically 60 cards, about 40% of which are lands. The rest can really be anything.

I would caution against buying actual cards unless you know her collection inside and out or she's been talking about some card that she wants specifically. The issue is that, aside from Basic Lands, you can only have 4 copies of any card in a deck at maximum. So if you get her something she already has 4 of, it's pretty much useless except as trade bait.

Your safest bet would be to find some place that really deals in Magic cards and get a gift certificate. Usually these are specialty stores that also deal in role-playing books and or comic books. I know gift certificates aren't great, but otherwise you are risking doing something comparable to buying a DVD she already owns.

A good magic deck can cost a lot, the pre-built decks they sell are usually mediocre, and you have to find out what format she's wanting to play. Most tournaments only use "standard" format cards, which means they were released in the past few sets. Older cards are much more expensive, and you can't use them in most tournaments. If you want something tournament legal and prebuilt, check the local stores for Event Decks. They're usually alright, and you can modify them to make them really good. Most of the time, I use around 24 lands in a 60 card deck -- 40%.

20 land/40 monster/artifact instant etc ratio

can be changed

How are you going to play? If you're looking for something to play together, I recommend the Commander format.