Hello I have a new bootrom 3gs that I jailbroke with redsnow in a tethered mode and installed 6.15.00 . is it possible to get it into a untethered mode I tried greenpoison over that jailbreak and it put it into some kind a continual boot mode where it just kept booting and shutting down . now im back to just tethered boot and all else seems good . please help

What's the firmware version installed on your iPhone?
Assuming you have SHSH for 4.3.1, create a custom firmware with PwnageTool or sn0wbreeze (Both have been update with the untether jailbreak), restore and you're done.

Actually since I last posted the iphone took a turn for the worst and started rebooting continually its a long story but the end result is went to Apple store and they gave me a New one under applecare gotta luv it. Now I dont want to do this again ,so I have a NEW 3GS 4.2.1 that I need to Jailbreak and Install 6.15.00 what is the best way ? Thanks for the help

If you want to continue on 4.2.1, use redsn0w to jailbreak and install iPad baseband.

Redsnow is what got me into trouble before I jaibroke and added ipad baseband but it was tethered so then I ran greenpoison then all hell broke loose i think I should have done greenpoison first then installed baseband what do you think

The problem is the combination you made. I suggested redsn0w because it can also install the iPad baseband, unlike greenpois0n. You can also use PwnageTool to create a custom firmware with the iPad baseband, but it takes longer than redsn0w.