Formal:
* Estou a chegar. (Literally: "I am arriving.") This is the most common and neutral way to say it.
Informal:
* Estou chegando. (Similar to the formal version, but slightly less formal.)
* Já vou. (Literally: "I'm going now.") This implies you are about to leave.
* Estou vindo. (Literally: "I am coming.") This is another common way to say it, slightly less formal than "Estou a chegar".
* Vou aí. (Literally: "I'm going there.") This is a more casual way to say it, and it implies you are going to a specific place.
Other options:
* Vou te encontrar. (Literally: "I'm going to meet you.")
* Vou para lá. (Literally: "I'm going there.")
The best way to say "I am coming" in Portuguese will depend on the context of the situation.