Iphone 3gs jailbreak problem

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I got given a Iphone 3gs a while ago when I first got it was locked to a uk network. I then got it unlocked while I was visiting the uk. The guy installed ios 4.0.1 on which is not great because I cant install any apps. I then decided having a go at jail breaking it using redsn0w 0.9.6b2. I been looking for some advice about what I should do and step by step guide. This is the first time I am using a iphone.