Go visit your professor's office hours and talk to them about it. Odds are they'll know someone in the field that needs help, or they know what companies have partnered with the school for internships. The same can be said of visiting your academic adviser. Find some local companies that you think you may want to work at and browse their job boards, they may have openings listed. Lastly, go to your university's career fair and talk to the various companies represented there. Express your interest that you want to intern and see if you can get started on the application process.
Speak with your counselors and also look for online postings. Internships fill up fast so be proactive and look well in advance
Many universities have e-mail groups per group (psychology, computer science, biology, etc) that send out information for opportunities, such as internships. I would look to see if your path of study has one and sign up. If you already know companies you would be interested in working for in the future, you can also check their hiring websites. Internships are usually listed as such, allowing you to apply!