What do I use to jailbreak an iPhone 3G? I just bought a used iPhone 3G and I want to jailbreak it.

What tool do I use?
You may try Spirit

http://www.spiritjb.com/

I support "any iPad, iPhone, or iPod touch on firmware 3.1.2, 3.1.3, or 3.2 (not 3.2.1)".