I have a Verilog-A code snippet for MOSFET, but I do not know where (any good free Verilog-A Simulators) and how to simulate the code to get the VI chara of the device. Could someone please explain the steps?
You are likely going to have difficulty. All major commercial simulators support Verilog-A, and the compilers are quite good. If at all possible you should go that route. NGSPICE includes the compiler ADMS, but the latter has been unsupported for quite a while, handles only a subset of Verilog-A, and is not simple to use. If you would be able to get your code running at all I believe it would take a significant effort. By far the easiest path is to use a commercial simulator.