Ain't is a contraction for am/are/is not or has/have not. It is technically grammatically incorrect and considered to be non-standard English, however, it is still a speaking pattern seen commonly in some parts of the United States. The no here does not actually mean no, it functions as an extra point of emphasis here. Ain't no preacher here can be understood as is not a preacher in this context.
Ex: I ain't no snitch. (I'm not a snitch.)
Ex: Ain't nobody here but us. (There is nobody here except us.)
Ex: We ain't got no options now. (We don't have any options now.)