Share 'Cut straight when edges aren't'

Any suggestions on cutting plywood (Circular Saw) straight when the edges aren't? If I use a straight edge guide and the edges aren't straight, how do I find another straight line to refer it to? I am a beginner and I am enjoying my new hobby. I just want to learn the tricks of the trade to make my projects worthwhile. The detail work (straight boards) are so important and I am lacking in this ab…