"I am thirsty. I want some juice."
Translation:Minulla on jano. Haluan mehua.
Jotain mehua is not really wrong - but it would be used when you are choosing of different juices in a shop for example, when you can see the choices in front of you. When you say you want "some" juice, then you would typically say "haluan mehua" or "haluan vähän mehua" - I want a little bit of juice....