Oh god, I remember struggling with streams when I started as well. Streams shouldn't exist. Jump justice.
Anyway, there are three ways streams can be hard: 1) you can't tap them consistently, 2) you can't aim them consistently or 3) you don't have the stamina to tap for so long.
Number 3 probably won't be a problem at the start, unless you begin with weak fingers, but it will definitely be a problem if you try the Freedom Dive beatmap you linked to.
I think it's easiest to begin by practicing on what are called "stacks". Stacks are basically streams that don't move, so they're really easy to hit with your cursor, but you still have to learn to tap them to the rhythm. Um, I actually don't know of any good beatmaps with only big stacks, sorry. Alternatively, you could use the
Autoplay mod (press F1 in the song selection menu) with any beatmap that has streams.
Once you know how to tap streams to the rhythm, you can turn off Autoplay and try to follow the stream at the same time as you tap them. I didn't do anything more sophisticated to learn streams, so the same method should probably work for you.
Oh, and
Jesse1412 linked to (in the post Velura linked to) a spreadsheet of stream practice maps you might find usefwl:
Practice Maps - Short Streams.