Very often this creak means the bb needs to be tightened and/or is dry. You'll have to remove the cranks, which requires a special tool. Check to make sure both bb cups are tightened--this also requires a special tool. Since you have the cranks off you might as well pull the bb wipe down the shell and cup threads, regrease everything and reinstall the bb. Remember that the drive-side bb is reverse threaded for most bikes (English/Standard).
Less likely, but much easier to remedy, are the pedals needing to be greased. Take 'em off, grease the threads and reinstall.
If you don't have the tools, space to work, or inclination to this yourself you might want to have a shop do this. But, if you're interested in doing this type of stuff yourself you'll need a crank puller and a bottom bracket tool. Check the parktools website or sheldon brown for more info on pulling your cranks and bb. This is something you can definitely do if you want to, you just need the correct tools and a good set of directions. If you have a favored shop buy the tools from them and bring your bike so they can make sure you get the right tools for your cranks/bb. You'll probably get a tip or two in the bargain as well.
Of course, it might not be the bb, but it is very likely from your desription. Besides the bb and pedals this could also be a lack of grease on the seatpost-seattube contact area and I've seen saddle rails/seaptpost clamp contact point also creak.
But, as I said before I highly suspect this is a bb creak. Just mentioning the less probable options because I can't make a certain diagnosis without the bike at being at hand.
Loose chainring bolts could cause your problem but I don't think it is probable.
Good luck, let us know if you have more questions.