My 1st thought is its probably a bad battery.
I would have it tested with a load.
Even a brand new battery can test bad.
Jump it with another 12v battery, and youll quickly be able to determine if its a bad battery.
Short across the solenoid's (start relay) large lugs with a screwdriver - ignore the sparks... If it turns over then the solenoid is bad, If not, then the battery is bad/undercharged, battery connections or cables are dirty/loose/corroded. starter has failed or engine is seized. My bet? Battery or cables.