I recently have started to play Magic Online (MTGO), which is basically just a digitized version of MTG. It cost me 10 bucks to get an account, and I bought one extra pack of cards, so I have invested about 15 dollars so far, which feels like a fairly low entry fee considering how much I've spent on physical cards. The client you play on allows you to buy cards from the main store, trade with other players, play casual games in various formats (I only play Standard), and join online tournaments.
When I first started playing I had to figure out what deck I was going to try and make, since cards essentially cost the same as their physical counterparts, and there's no realistic way to get digitized copies of cards I already own. So with the cards I started out with I began looking at trading bots that are run by players, when you start talking to one it is similar to downloading files from an IRC bot:
It took me quite awhile to hunt through a bunch of bots to find the cheapest versions of all the cards I need, but once I was done I had constructed a cheap version of my RDW deck that I talked about in my last post. (pretty much identical, just missing about 8 rare cards that I didn't want to spend the money on)
Once I had my deck constructed, I started joining casual games to test it out, below is a screen shot of the casual game finder window. One of the nice things about MTGO is there are hundreds of players online all the time, so it's really easy to play test a deck.
Once in a game you just click through the phases (playing cards and using abilities where needed) as the game progresses. Below are some pictures of me stomping on a random player pretty hard, and an example of my RDW deck winning on turn 3 (the fastest that it can win).
