I bought a second hand iphone 3g now asking password, what can i do now?

you have to master reset and or factory reset or you can add new I cloud and use

You can put the iPhone into recovery mode and restore it using iTunes. Follow the instructions on the following website:http://osxdaily.com/2011/01/08/iphone-recovery-mode/

1. Turn on your Apple iPhone 4 with an unaccepted SIM card.
2. The iPhone should now request the unlock code or request a password of some sort.
3. Now enter your unlock code.
4. Your Apple iPhone 4 may reboot and should now be completely unlocked.
Also keep in mind that if your Apple iPhone is not a GSM cell phone, then these instructions will not work. However, I am pretty sure that at the moment only GSM or SIM card based iPhones exist. Anyway, good luck unlocking the new Apple iPhone 4G and just leave a comment below if you need some additional help or tips.
Note: The free unlock instructions provided on this site only work with GSM or SIM card versions of the Apple iPhone 4. As always there may be already unlocked versions of this mobile phone, but generally cost a lot more then unlocking it yourself.

You must connect to iTunes and set up your AT&T plan/info

iPhone setting:*
- Auto-Lock: 1 Minute;*
- Passcode Lock: On, require passcode immediately, after 15 min or 4 hours etc;*
- Network: Turn 3G Off

Scenario 1: Home screen is active with screen on*
1. Press power button for seconds until asked to '"slide to power off"
2. Slide to power off
3. press power button to turn on iPhone
4.*When asked to enter passcode, touch the "Cancel" key, then slide to unlock
5. bypassed passcode

Scenario 2:*
1. On the screen of being asked to enter passcode, press power button for seconds until asked to '"slide to power off"
2.*Slide to power off
3. press power button to turn on iPhone
4.*When asked to enter passcode, DO NOT touch any key on the screen, press power button for seconds until asked to '"slide to power off"
5. Slide to power off
6. press power button to turn on iPhone
7.*When asked to enter passcode, touch the "Cancel" key on the right bottom corner, then slide to unlock
8. bypassed passcode

Notes:
1. When Network setting changes to Enable 3G: ON, "3G" signal shows up, passcode CANNOT be passed through
Go to a underground parking lot or a lift where 3G signal is weak, when 3G signal switches to "E" (Edge), passcode CAN be passed through following above steps
2. When no 3G or Edge signal, above steps don't work

Conclusion:
Seems passcode security is related to network status
Possible security hole under Edge

First put the phone into dfu mode by turning the phone off.
then hold the power and home button at the same time for ten second the release
the power button and hold the home button for around ten seconds until itunes recongnizes and iphone in recovery mode that needs to be restored,
then restore from whichever firmware you want :).

The iPhone unfortunately does not pair with just any device, it would be best to check compatibility before purchasing a new peripheral for your iPhone.

yes you can jailbreak your iphone and upgrade to firmware 2.2 this will unlock your iphone to any sim card and shoud enable your gps again ask this bloke at the link click here he is a genius with everything to do with iphone's

No  you can not fix that problem it's just the way the iPhone 3G was built. If there is anything else I can help you with please feel free to ask me.
Thank You, Apple Care Service And Support Stacy R.

It sounds like the handset is iCloud locked (find my iPhone hasn't been turned off). This happens too many second hand phones, the previous owner isn't aware of this or u have bought a stolen phone?
If you know the previous owner they would know the apple ID and ID password to activate it, after that you would have to do a reset the phone.
Good luck