What would you do if someone just left their newborn baby on your porch step because they didn't want it?
I'll pick the baby, Try to find someone "Another" family member, it should be someone to Find so, if I couldn't I'll go for Police station, or Baby Center, & I'll explain the position then They'll decide
call a hospital / police .
If able, I would take care of it.....If not, I would do my best to find it a suitable home. At least whomever left it (no matter how wrong it may be) didn't kill the baby, as some sickos would.
Since I live with my parents I'd leave to them, but if I was living on my own I might try to take care of it