Keep it brief! In general, an Introduction section of less than 500 words is ideal.
Include:
~a very brief review of the field to introduce non-experts to the topic
~the major references (a few historical and recent reviews) related to your topic
~ideas or areas in the field that have not been characterized or thoroughly investigated
~an introduction to the present study, including a rationale for the study and/or the models used (what information will this study contribute, how will this study be different from previous studies)
~any previous reports about the precise topic you are investigating (and why your information will be better or different, e.g. larger population, detailed mechanistic studies, etc)
~any other reports that you have published supporting that you have experience in the field
