Starcraft 2 is honestly one of the best games ever made. And that's saying a lot for a sequel. Most sequels never quite get it just right. They either lose the feel of the original or are just plain silly. Movies too, but with starcraft 2 Blizzard was able to get a game that is cutting edge with a ton of changes that still feels exactly like the original game did. And that's not easy to do. And perhaps one of the coolest things you can do now is buy starcraft online, an option that just wasn't available even just a few years ago.
Even just five years ago high speed internet access was a lot more iffy, not to mention slower. But as that kind of technology takes some tremendous leaps forward, it has ushered in the possibility of buying games, downloading them and never having to mess with driving to the store and using the cd's or dvd's in the box. It's still going to be a lot faster to do that, unless you live like 500 miles from the nearest store.
So really all you have to do to buy starcraft online is go to battle.net and create a free account there. Once you have a battle net account you can buy a game key right from the interface and get the installer for starcraft 2 in just a few minutes. Now, the game download itself is absolutely massive, so be prepared for a wait - probably a long wait. Unless you're on a T3 connection or something it's going to take the better part of a half a day, or even several days if your connection is wonky.
You can also buy starcraft 2 online from other vendors, but let the buyer beware. there are some very good game sellers out there that have good customer service and slightly better prices than Blizzard itself, but you're also opening yourself, and your computer up to heaven knows what. It may cost you a few bucks more, but at least with Blizzard you know what you're getting and you can be sure you're getting it without viruses and sypware and any other little nasties you might potentially get from other sources. We have used vendors like _________ before and found them completely reliable, however if the few bucks is important to you.
And that's all there is to buying starcraft online. It really couldn't be any simpler, although it could be a lot faster. But that's not really Blizzard's fault after all, we'll just have to wait another five or ten years until connection speeds (hopefully) can catch up to the sheer size of these games. Perhaps that internet 2.0 we've all been hearing about will help by getting rid of the whole packet protocol, but that's a little ways off at least. But for now, this is what you're stuck with, but at least you can buy starcraft online right now if you want to.