How can I put my own passcode in?

Please see Set a passcode on iPhone - Apple Support

If the phone is currently locked by a passcode you will have to enter that before you can proceed. If you don't know that passcode you will have to erase the phone - If you forgot your iPhone passcode - Apple Support

What passcode do you want to change? If you are asking about the passcode to unlock your phone, you set that up yourself. If you want to change that one, you can do that in the settings app.