It depends on what type of opportunity you're looking for. I got my major research opportunities through looking through my school's faculty website in the department I was interested in.the websites usually listed their research projects. I then emailed professors that I wanted to work with explaining my interest and credentials with an attached website. If a researcher had room they would set up an interview to discuss my interest and goals.
For a summer research experience you can do a google search of "Summer Undergraduate Research Fellowship" which should provide you with links.