1.) You buy singles from bots. You can find them on the marketplace in MTGO, and you pay them in Tickets (bought from the Magic Online store). If you want to use a credit card, there are several sites that sell MTGO cards- I like
mtgotraders.com- which after buying the cards from the website, will contact you in-client and "trade" you the cards you bought.
2.) Mtgtop8.com is a simple site that aggregates decks from tournaments if you are just looking for raw lists, and strategy sites like Channelfireball, Starcity, TCGPlayer all have articles talking about decks.
3.) You can play for free in either the casual room or the tournament practice room as much as you want at any time of day. In addition to scheduled tournaments, there are also 8-man constructed tournaments that fire whenever they get 8 players, and 2-mans that fire as soon as a second player enters the queue. Those all cost Tickets.