Over at The Joy of Programming there’s a nice introductory post on using Graphviz’s dot command to draw graphs and how you can make the process almost interactive by calling dot from within an Org source block. The dot command language is pretty straightforward, at least for relatively simple graphs, so it’s easy to learn.
I almost always use dot when I want to draw a graph. There’s an excellent manual that shows how to make arbitrarily complex graphs. Page through the manual to get an idea of what you can do. If you don’t use dot very often you may forget the details. For simple graphs there are tools that make the process easier. One example is Org Mind Map that I wrote about the other day. It draws the graph from the structure of an Org tree. That may seem limiting but as the examples show, you can make reasonably complicated graphs with it.