Designing a new bridge can be very difficult, requiring years of experience and education. However, there are already many millions of bridges built around the world and you can just look at how they are made and copy them for the most part. You can look at other structures as well that aren't bridges for ideas. Consider the structure of a high tension power line tower, or a radio tower. Look at metal scaffolding on a building that is under construction. They all have combinations of rods arranged into geometric shapes like triangles and squares. Or think about suspension bridges, those are constructed to span long distances using wires to hang a flat bridge on. Look at this web site for more examples and information, good luck!
How bridges work