The Office of Career Services at TC helps you improve your resume and cover letter, they offer to do practice/mock-interviews, and take free professional profile pictures.
You should start looking in January for summer internships, when companies start allocating their new budgets on different departments. However, internships are posted year-around. Summer is a particularly popular time for completing internships and therefore more competitive. Some companies post summer internships as early as fall. So, don’t delay, start looking now!
If you don't find the right internship here, check out Glassdoor, LinkedIn, Indeed, Idealist, and company websites.