How to design a simple linear DC circuit in Ngspice?
There are some simple example netlists in the \Tests folder. 

A DC voltage source would be specified as:

V1 1 0 5

Where V1 is the name (could be V2, V3, etc) 1 is node 1, 0 is circuit ground and 5 is the voltage.

Here is a basic circuit to try, a voltage source connected to a 1 ohm resistor and stepped from 0 to 10V with current plotted for R1 (not tried it but it should be okay, let me know if not)

** Circuit Description **
V1 1 0 5
R1 1 0 1  

*.option noacct
.dc V1 0 10 1  

.plot i(R1)  

.end  

It's probably not the easiest SPICE to pick up, you might want to try LTSpice.