You need to focus on the more technical courses during the college years, like structural analysis, geotechnical courses or highway design, which ever specific area is of interest. Try to land internships in different places if possible to get a better idea of what is the best fit. Then after graduation look for positions in design group, at consulting firm. Government agencies often don’t have a large in-house design work load so be careful about which place you choose to start with. It is often hard to switch from field work to design after a few years because the salary of field experience doesn’t transfer to inexperienced design engineer. It’s easier to make the transition in the other direction.