Was a duel deck a good first purchase or is an intro deck better? What about commander decks?
It's pretty ideal first purchase as they're cool decks that are usually fairly well balanced so you can play them against a friend. Commander is a great format for casual play too, it's my personal favourite. There's some cool pre-made decks about that easily lend themselves to upgrading when you feel ready.
What type of deck would i actually be wanting to create to just casually play over a few beers and some friends? I cant understand the whole legacy/modern types could anyone clarify that for me?
Don't worry about formats if you're just playing casually against friends. That being said, don't build something so broken it blows your friends out of the water either. Basically, each format has different card pools available, Standard has the most recent 6ish sets, Modern has way (50ish) more and Vintage and Legacy have basically every card ever printed.
How do different expansions (is that what they are called?) come into play? Is one released every year and the previous one phased out and becomes irrelevant? Or is that only for tournament players and in my case any card will work with my deck for casual play?
When a new set is released (~every 3 months), it's cards get added to the card pools available to Vintage, Legacy and Modern. Standard is a little different, in that it sometimes knocks out a previous block. Here's a gif of how the next few sets will affect Standard. For casual play, generally anything goes, you don't need to worry about sets, blocks or formats really.
And lastly, for a casual player how to i expand my collection? Booster packs? I read all over that they are not worth it. If i have, for example a red deck, do i just value any of the red cards i find in a booster?
The most cost-efficient way to buy cards is as single cards, the ones you need for any given deck. That being said, there's nothing wrong with buying boosters but to get the most value out of them look into playing draft or sealed with them before cannibalising them for decks.
Hope that helps! :D
