みなさんこんにちは!

私はらら／ララです!　※Katakana is used to write foreigner's name.

私は15 歳なのですが!

私は猫がすきだよです!　※You should use the same word style. だ is a normal style and です is a polite style. よ is a particle to emphasize. It's used when the speaker/writer wants to inform the sentence to the hearer/reader firmly. This is a usual self-introduction. The hearer/reader has not said that you don't like a cat. He/She has not asked whether you like a cat. So you'd better not to use よ.