Good evening Rosa,
Great that you want to apply for a position such as this as you can with your interest and background make a difference to those that need it. As for a cover letter, I went ahead to link to an example of one that you can use as a template as go ahead and use as an example and then feel free to send to me to review. I will be happy to look it over to see if I can suggest any changes if needed. Here is the link:
Looking forward to hearing back from you.