my iPhone 5 does not have a passcode and has been lost/stolen. How can I activate "find my phone" if it is one the phone? How can I lock the phone down so it cannot be used?

Sorry but Find my iPhone needed to be activated from the phone itself. If it was not on it is now too late. You cannot lock it now. Report it stolen to your carrier and to the local police.

If FMiP was on follow the steps here:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

In addition to what elcpu already said, change any and all passwords immediately. Since you said your device did not have a passcode lock on it at the time it was stolen, the theif has full access to anything and everything on it. Any and all "lock down" and security features needed to be initiated while you still had the device in your own hands.

And please, in the future, with whatever phone you have, activate and use a device passcode lock, and device activation lock feature. You've just handed a thief a ready to use device by not utilizing the installed and available security features. Not to mention that all your personal information on that stolen iPhone is now available and accessible to the thief.