An idiots guide to buying and selling BitCoin:
It's actually really easy.
It's just not as easy as it would be if PayPal / credit cards were generally accepted.
The reason most such things aren't accepted is down to whether they trust you to buy a currency with a currency, issues around chargebacks vs. guarantees they have the funds, and so on.
The simplest way to buy and sell is via an exchange such as Mt.Gox (mtgox.com). This one, for instance, handles 80% of all BitCoin trades.
You can fund your account via a number of other virtual wallet accounts that make you want to self harm by the time you've got money into them. Or, much simpler, you can make a wire transfer from your bank account. I've transferred money in from my normal online banking to their account details in Japan before going to bed one night, and had it credited to my account and fully ready to spend by the time I've woken up.
You then just treat it as equities trading: there are buy and sell prices, and you put a bid and a quantity in.
So, if you wanted to just buy a few on the off-chance they end up being worth trillions, you can.
Caveat: if the exchange you're holding them in gets hacked, they could "disappear". So, the idea is that you keep them somewhere trust-worthy. How you can keep a set of numbers safe is an interesting one! (Printed out, in a safety deposit box??) You can "download" them to your computer (sorry - explanation of how to do this without the footprint still being at the exchange is too tedious to go into) - IF you trust yourself to manage them better.
Now, it gets more interesting: Different exchanges have different buy/sell rates, so there's some real arbitrage fun to be had buying on one and selling on another. And once your currency is in BitCoin, moving it between the exchanges is VERY easy - not like the initial wire transfer
As for selling them - Wire transfer back out.
One other option is "physical" purchases of them. You can trade duo-tone pictures of the Queen's head (or dead-pres's) for them with quite a few people. And there are sites for that, too. E.g: https://localbitcoins.com/location/GB/London/
Other ways of getting them: mine some. Although, that's getting to be a much more commercialised operation than ever before, so unless you have a server farm going spare, or a half dozen top end GPU-laden graphics cards to throw at it, not such a great idea. (For any large-scal sysadmins out there, having a screen-saver that coin-mines could reap large rewards).
And there are sites that'll give away tiny fractions of BTC for clicking links and the like if you just want to have some kind of [tiny] holding of them on the off-chance they become rarer than common sense in a currency bubble.
If you get into mining them, buying and selling for cash on the street, or link-clicking, you'll need a wallet to hold them in: http://bitcoin.org/en/choose-your-wallet
If you're just trading them on Mt.Gox, that's not necessary.