Hi! Bridge designer here :) Depending on the span, you should look at either a girder system (one big beam) or a truss system (multiple beams oriented to form triangles.) You could also experiment with adding piers and struts, if this program allows it. I would suggest using Google Images to look at different bridge styles - others include suspension, cable-stayed, and concrete box beams.
If you are really serious, consider looking at the AASHTO LRFD manual, and the FHWA website.