Hello Kira,
There are a thousand different opinions, but modern thinking is you start from the ground up. Which means see if there is a nice Judo or BJJ school in your area. Preferably one with a kids program that promotes competing in tournaments.
Not that you have to compete, it just shows that the school is a part of the larger Martial Arts community.
Of course there are many other great arts to chose from as well, but you will never be sorry for learning a good base like Judo or BJJ.
Good Luck.
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Undefeated in all of Asia!