Advice on Iphone 4 PLEASE

[Newiphone27]

Hi, I just purchased a iphone 4 that is stuck on "enter valid sim with no pin lock" screen. The guy said he thinks it was on iOs 4.2 because it was bought a few months ago. He also said it was a 32gb. How do I check what firware, version, baseband and model this is? Can I jailbreak this phone no matter what version and firmware? If so what tools do I use? Please give me a simple step by step or link to it.
Thank You all very much for your help and support

[iYeow]

You can use iforecast to see what firmware you have. You need a valid sim to trigger into home screen.

[Newiphone27]

can I use a ATT prepaid sim card to do this?

[MrDMac88]

If I'm not mistaken, u can use ANY ATT sim

[iYeow]

can I use a ATT prepaid sim card to do this?

Yes, you can.

If I'm not mistaken, u can use ANY ATT sim

Yes, you can use any atnt sim card, even those that does not have service will work.