Get an Abit (IS7, IS7-G, IC7, or IC7-G) or ASUS (P4C800/E or P4P800) motherboard, or read
this AnandTech article and pick the one you like the best.
To overclock a P4 2.4
C, make sure you have a good power supply (like an Antec True 480 or something) and good memory (like Muskin, Corsair, OCZ, or maybe even Geil, HyperX, or Buffalo -- get PC3500 or 3700 with good timings), then set your FSB to 250, set your memory ratio to 5:4, and set your PCI/AGP speeds to Fixed 33/66. Wha-lah -- a 3GHz processor. Run some tests to be sure the machine is stable (Prime95 / 3dMark '03). Then inch up the FSB 5MHz or so at a time, running stability tests each time. When the computer won't boot or won't run the stability tests without rebooting, you can either go back to the last stable speed, or start playing with the voltages to try to get the processor and/or memory to run faster. Since you apparently haven't done this before and since you probably will be using stock cooling, you might not want to mess with the voltages. That's when you start to risk serious damage to your components. Make sure you do some reading before you start changing voltages.
Remember, overclocking means running your parts beyond their rated speed. If you blow your box, it's your own fault and no one else's (especially not mine). You have been warned.