I have a pain in my anus at the right side and I want to know the cause and how to treat it. Thanks

Impossible to tell from such scant information.
You need an examination from a health care professional.

It would help if you could see it, or have someone look. If you can't do that, feel it yourself delicately in order to get a sense of what might be unusual, and report back what you feel.
The most common causes:
Hemorrhoids that are falling out
Fissure (split skin)
Pimple or cyst