How do I get into an old Apple ID

See If you forgot your Apple ID https://support.apple.com/en-us/HT201354

And, see If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid 

And you will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485

Before trying to answer your security questions is very important that you think about your questions, including your birth date, and their answers carefully and try to recall all details you can. If you don't answer them correctly enough, then you easily can lose control of your account.

Note that If you're using two-factor authentication and can’t recover your password, you usually willneed to enter account recovery. See Recover your Apple ID when you can’t reset your password  https://support.apple.com/en-us/HT20492

And, in general, if things go badly using the above links, and I think they will, you will need to try to recover your account if you want to keep using it. Again, see Recover your Apple ID when you can’t reset your password https://support.apple.com/en-us/HT204921