Assuming you understand the convinsion y=f(x)=sin(x) for example and what this means, you can choose values for x and compute the corresponding y values then plot the (x,y) pairs on a chart. The more close the x values are, the better your plot would be.
The above is the simple way. You can try to get more sophisticated and calculate the function max. and min values as well as where the function intersects with x and y axis to make sure your sample x values pass through the critical points that gives this function its distinct looks.
You can also use a calculator or a web page that would help you visualize the plot. Here are some links for you:
There are other tools to allow you to plot functions that has more than 2 variables.