The easiest way to experiment with Asterisk is to grab a VMWare image of Trixbox. This won't allow you to interface directly with land lines, but will let you get your feet wet, as well as allow for setting up some soft phones or hard phones if you have any without needing to buy anything.
If you have a machine to dedicate to it, I would suggest you start with Trixbox and install it on a machine (it reformats the hard drive). To interface with a PSTN line, you will need to purchase some hardware. The cheapest method is using a X100P clone card. You can find more information about it on
voip-info.ofg under the "X100P cards (including clones of the original Digium Wildcard)" section. I have purchased a couple of these cards for only $10 each, but they are not production quality. They are very sensitive to the lines they are connected to. I had them working perfect at my house, but when installed at my friends store, they were so full of echo they were unusable.
If you need to have something more professional (of higher quality), expect to spend around $150 - $200+ for an FXO card depending on how many lines you need.