Formal:
* Hi yw hi'n gantores. (This is the most literal translation and is grammatically correct, but it's a bit stiff.)
Informal:
* Mae hi'n ganu. (This literally means "She sings", but it's understood to mean "she's a singer".)
More specific:
* Mae hi'n gantores opera. (She's an opera singer)
* Mae hi'n gantores pop. (She's a pop singer)
You can also say:
* Hi yw'r gantores. (She is the singer)
This would be used if you're talking about a specific singer who is known.