how do I set up an old iPhone I've used before on my account

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

You may --> Set up your iPhone, iPad, or iPod touch - Apple Support (IN)

The link on top of this page, under the "Related Article” is the process. If that link did not help then please mention the exact difficulty faced by you.