Formal:
* Je suis une Française. (This is the most common and formal way to say it.)
Informal:
* Je suis une fille française. (This is more casual and emphasizes "girl".)
* Je suis française. (This is even more casual and just means "I am French".)
You can also use the pronoun "on" to refer to yourself in a general sense:
* On est une fille française. (This means "We are a French girl," but it's used to express a general truth about French girls.)
The best way to choose which phrase to use depends on who you're talking to and the situation.