Formal:
* Je suis à mon domicile. (This is the most formal option, often used in official settings.)
Informal:
* Je suis à la maison. (This is the most common and natural way to say it in everyday conversation.)
* Je suis chez moi. (This is another informal option, meaning "I am at home.")
You can also use the verb "rester" (to stay) for a longer duration:
* Je reste à la maison. (I am staying at home.)
Choose the phrase that best suits the situation and your level of formality.