Hi Juliana,
I hope the following format will suffice:
The Manager / Officer / etc
Name of the college
Place.
Dear Sir/Madam,
We are glad to inform you that based on the tests (mention the name of the test or if an interview has been conducted, the date of the interview), you have been selected for an internship at our office (address of the office). Your stipend will be [mention the amount that you are planning to give] as per the discussion we had. You will be reporting to [mention the name].
Thank you,
XYZ Company
Name of the HR/Authorized Person
I hope the above will be sufficient. If in case I am wrong, I would welcome other suggestions from the friends here.
Warm Regards,
Julie