I would recommend starting in OU before moving to Uber.
OU is by far the most played metagame, you'll see the good, the bad and the ugly strategies,
sometimes you'll see that your favorite pokémons aren't as good as you once tought,
sometimes you'll start using strategies you'll once tought were despicable,
but you'll have opportunity to see a lot from your mistakes and incorporate good strategies to your teams,
you'll learn to see the difference between experienced players from the ones who aren't and
will have a great opportunity of growth, thing that wouldn't be as good starting from Ubers.