So what happens when you try to boot it? Is it by any chance asking you to provide a Google username+password? If it is then you need to provide the login credentials of the last Google account used with that phone.
This is an anti-theft measure known as "factory reset protection". Prior to its introduction it was possible to steal a phone, go into recovery mode (without needing to unlock the screen) and reset it, then use it or sell it. The idea is to reduce the attractiveness to a thief by making that impossible, but of course it will also catch people who forget the password or who get a phone legitimately from someone who doesn't know about this (if you want to sell a phone you should remove your account from it before resetting in order to avoid this).
The catch is that it only works as an anti-theft measure if there is no way of getting into the phone without the login details. So if this is the problem you are facing we can't help you: my advice in this case is to speak to the person who you got the phone from and ask them to unlock it (or provide the information you need to do so yourself).
If the problem is something else, could you describe exactly what happens when you try to boot it up and log in?
