Sometimes getting started is the biggest challenge. It may help you to know that all writers, even experienced ones, sometimes have difficulty. It is useful to develop a repertoire of "prewriting" strategies for getting started. For some tried-and-true techniques, follow these links:

Reading Toward Writing
Tips for reading when you will have to write about the text, from the University of North Carolina at Chapel Hill

Finding a Topic
Some suggestions for finding a topic, when your writing assignment gives you a choice of subjects.

Choosing a Subject at Paradigm Online Writing Assistant
Check out this excellent online writing manual. This link will show you several strategies for getting started.

Purdue University's Prewriting pages
Follow the links for advice on getting started at the original OWL.