I have iphone 5,i have changed my password in yahoo but now when i try to use my phone it is showing incorrect apple ID..Now my phone is showing activation lock

Does your screen look like this (with the ID being shown not yours)? -

If a second-hand iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it) - please see http://support.apple.com/kb/ts4515

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.