First you have to choose the bridge type that you plan on building. A majority of people would choose the truss bridge. From there you have to choose what type of truss bridge you want. Depending on what you want, you might choose the Warren Truss bridge if you want it to be light or the Warren Subdivide Truss Bridge if you want it to be strong. After that you can do more research on the type of bridge you do. After you are done researching, it would be a good idea to draw out a design for your bridge, so you would know what you want to build and how to build it. From this point, you can start building. It is a good idea to start early, since the glue needs to set and you have time for modifications.