Check out methods of setting up and using an "Alaskan sawmill". You can buy them as an attachment for your chainsaw, or you can build your own. With a little practice they can make decent dimensional lumber. It will never produce as smooth a finish as a bandsaw or circular sawmill.
If you don't want to work that hard, just get a contractor's chalk line and pop a straight line onto your log. Make a few light passes with your chainsaw following the straight line you marked. Once you have a straight line etched deep enough in the log, use that line to cut off the slab that you laid out.
I have used that technique to make replacement boards for my trailer. It works pretty good with a bit of patience, but not nearly as well as the Alaskan sawmill.