As you can see there are only 2 indefinite articles. Male and neutral nouns share "ein", while "eine" is used for female nouns only. Girls really like to be special, so they got their own article! Too bad that even potatoes and cucumbers are female in German while girls are actually neutral. (You can find further explanation concerning "grammatical gender" at the last half of this article)
You can also negate the indefinite articles which looks like the following:
Ich habe eine Katze. - I've got a cat.
Keine is not just used to negate female nouns, it's also required for negating the plural. Be careful, because the use of "kein" and "keine" is much more limited than the use of the English "no". There's another way you can put it.
I hope you'll enjoy your brandnew articles. Feel free to leave me a comment and have a nice day!
