First describe your program, that is, specify the inputs and outputs, for example
module my_project( input1, input2, output1 )
where input1, input2, output1 are the ports of your block for which you receive data. Next, you need to describe each of the ports, for example:
input input1; input [15:0] input2; output [3:0] output1; reg [3:0] output1;
I note that I specified one of the ports as a register. After this, a direct description of the logic of your program begins. You need to determine whether you are going to implement a synchronous or asynchronous scheme. If you are not sure that you understand what I mean, it is best to read various literature on the subject of hardware description languages, for example, various books from Pong Chu or Harris and Harris Computer Architecture.