Usually whatever company or organization you are applying to will have instructions on how to make contact. They may ask you to email a resumé and cover letter, submit them online, etc.
If there isn't a clear path to contact or if the company or organization you're interested in doesn't explicitly offer an internship, you may want to send them a short email detailing why you are interested, what you can bring to the company, and why the company is right for you. If the company or organization doesn't explicitly offer an internship, ask them if they would consider developing a position for you and explain why you think you could be a benefit. Perhaps even suggest duties which you would be qualified and happy to perform.
“I've learned so much inside the classroom, but my internships have helped apply that knowledge in a real and professional way”