Once you have found the right internship, look carefully at the instructions from the employer on how to apply.
Some will ask you to complete an application form. Others will want you to email a CV and covering letter.
Treat your internship application like an application for a permanent job. Tailor your approach to the organisation and internship you're applying for.
Be realistic about your skills and experience, but make sure it's clear why you're suited to the opportunity you're applying for.