problems turning over

I have a 91 4.0 5speed ill tell yall that truth im 18 and i kinda dont no anything about a explorer..like it wont start by itself i have a kinda battery and it still wont start..it turns over really slow..i have to jump it off still with a that battery..whats wrong..i was thinkin it mite be my starter..but idk..what yall think?