Forget old apple id

See if you can recover the ID/password.

Apple ID - How to find your Apple    An iCloud e-mail address is an Apple ID.

Apple ID - Find              

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage            

Apple ID – Forgot Password            

If necessary, if you are the original owner, contact Apple ID Support. Apple Support will need to see the original purchase receipt with the serial number/paperwork.

Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.  (Link doesn’t always load)   

Get Support      Work through the screens to get a chat option. 

https://twitter.com/AppleSupport

It sounds like you would need to use the new Apple ID, which you apparently know. That is, if you changed your Apple ID, then your old Apple ID would be irrelevant.

But, if you obtained you new iPad second hand, and there is an Apple ID in it you don't know anything about, then you may have been scammed into acquiring an iPad you won't be able to use.

The links below explain what may have happened to you.

Before you buy an iPhone, iPad, or iPod touch from someone other than Apple or an authorized Apple reseller, make

sure that the device is erased and no longer linked to the previous owner’s account. 

And see: Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365 See

this heading Check for Activation Lock before you buy a device from someone else.