Casual playing of Magic the Gathering MTG
This may be closed as opinion-based, but there isn't a &quot;right way&quot; to play MTG casually or to build decks. However, I find that you are approaching it from the wrong point of view, and are missing out on what is (in my opinion) the most fun part about playing Magic, which is the deck-building.
Sure you can find a deck list online that uses some of your cards and just build a deck that someone else has thought of. But then you are only getting half of what Magic has to offer... you are getting the play-the-game part, but you are missing out on the build-the-deck part.
My recommendation would be to avoid looking at any deck lists, and instead just look through the cards you have. Get some ideas based on the cards you have and then build the best deck you can with only the cards you have. Of course it won't be as strong of a deck as it would if you were buying whatever specific cards you need to make a great deck, but it should be balanced against the deck that your girlfriend made since it came from the same pool of cards.
It sounds from the question wording like you are thinking of a Magic deck as one of a bunch of different possibilities that you can find and use. But really a Magic deck is any set of at least 60 cards that doesn't contain more than 4 of the same card (except basic lands). So there's no &quot;right&quot; or &quot;wrong&quot; decks to make, there are just ones which will win more often or be more fun to play. But challenging yourself to build a deck without the ability to just use any card you can think of can be a lot of fun.
The one thing you may need to get outside of the cards you already have is basic lands, depending on what basic lands your collection came with. You can get a lot of these very cheap, but you will need several of them to build almost any deck.