To buy it you just go on the minecraft sight and buy it with paypal. It's $10 euro and thats about $13 US.
Once you buy it you own it and can dowload/redownload as much as you like. When you start the game up you sign in using your usename/pw and you good to go.
Welcome to Minecrack, try not to lose your gf/wife/job/lfe.
BTW the free version don't have survivol so now when it gets dark you better be in some kind of shelter with some tourches lol.