Welcome to the forum!
Let's start by finding out what model year your Explorer is. Also, does the Theft/Security light flash rapidly when you try to start the truck? If you turn the key to the Start position with the headlights on, do the headlights dim noticeably? Have you checked the battery connections at the battery to make sure they are clean and tight?
Do you have a multimeter to be able to take some voltage measurements?
-Rod