Many programs provide resources to help their students find and apply for internships in environmental science. If not, you should seek out an internship or some other type of supervised work experience on your own. An internship is the opportunity to gain hands-on real world experience in the area of environmental studies that most interests you. You also make professional contacts and improve your job prospects.
If you're interested in pursuing environmental science on a global level, you should take courses in a second language as well.
Research the location and faculty at different schools in order to match your research interests. For example, let's say you want to study freshwater ecology. In that case, you should look for schools that are either located near a freshwater lake or river or have faculty that also research this area. Schools often offer courses on environmental issues pertinent to their locations.