What u need are;
1) Casing
2) Mainboard
3) P4 CPU (INTEL Core2 Duo E4400 {2.0GHz})
4) DVD-RW 16x
5) Rams (Best if can go for 1GB Ram)
6) Floppy Drive (Optional)
7) PCI Express AGP Card (Optional if there's bulit in Graphic card in Mainbaord)

PCI Wireless Card (Optional if you have a wrieless Router)
P/S: If you're someone who do not know much about PC hardware, it's advisable for you to buy full set. Reason: When your PC spoil, can call for service instead of going to diff vendor to claim your warratny.