As mentioned a circular saw is needed for strait cuts, although as mentioned, learning how to use a handsaw is a good alternative. Speed square is a good addition as well. You could make saw guides that will serve you well. Take a look at http://www.youtube.com/watch?v=uCm07apw6-A for an example.
Make your own miter box. Its easy to build and will be orders of magnitude better then the plastic one you have. Check out http://video.renovateyourworld.com/v/21291653/how-to-build-a-miter-box.htm for an example.
You mentioned you had a plane, a shooting board will help fine tune your miters and other cuts as well. See http://www.youtube.com/watch?v=SsfBO42XweQ
You don’t need a shop filled with tools to produce nice work.
-- Nicky
