Check your BIOS. See if it has a setting where you can increase the speed of the FSB(front side bus, in MHz). Default will be at 133-200 mhz, and increase it slowly and stress test your PC with superPI, 3dmark etc after every increment.
Remember that increasing the FSB speed will affect your RAM too so check if your RAM has a high MHz (RAM rated at 800MHz and above should be good enough since your processor's default FSB is 533MHz). Also keep a check on your temps.
If you are a n00b, don't bother overclocking...too much hassle for limited performance.
Your PC is very old. You won't see a lot of increase in performance by overclocking a P4. I have a p4 and overclocking it didn't bring much performance difference (even after overclocking it by 400MHz). I also fried my RAM because it wasn't built to handle the increased FSB speed and since i was a n00b back then.
i'd suggest you buy a new pentium dualcore/core 2 duo/i3/i5/i7 PC.