Building experience is an important first step into many environmental roles. Developing your skills and knowledge, as well as making contacts, will help you in your future career. Volunteering is the starting place for many. You can start while you are at Sussex...
For ecology/science roles you can use our links for Researching Environmental Consultancies to find your own internships. Some may be advertised or you can approach companies speculatively.
You could also look at Graduate employers in engineering/construction as many of these will offer placements/summer internships, too.