How do I modify vBulletin?

[Hypern]

How would I go about creating custom modifications for my forum?

[KW802]

That is a rather broad question. What exactly are you trying to do?

[Brandon Sheley]

maybe this will help you along,

http://www.vbulletin.com/docs/

as KW802 said, this is a really broad question

[Dismounted]

Also have a look at the Articles section on vB.org.