First of all, let's try theses:

a) Open the Settings app and tap Sign in.

b) The next step is to enter your Apple ID and password, or tap Forgot password or Don't have an Apple ID? option.

On this screen, you can do the following:

  • recover your Apple ID/password;
  • create an Apple ID;
  • set it up later.

Tip: In case you have more than one Apple ID, tap Use different Apple IDs for iCloud and iTunes.

When you sign in with your existing Apple ID, you might be prompted for a verification code from the device you have previously used.

All in all, the process is easy and straightforward – you won't be spending much time to sign in into your old Apple ID.