If you want to used nulled version(cracked) of Vbulletin then go to sharemafia or scriptmafia and search for it.A lot results will come with download links.

If you have the money to buy then u can directly visit vbulletin main website and buy the scipt.Then after you downloaded the package there will be a readme file in that package which contains instructions of how to install it.

If you want help with installing i will do it.

There are also other forum scripts like IPB,Mybb(free),SMF eetc