I believe tu is informal and Usted is formal. For instance if you are talking to a friend or someone very familiar you use tu. If you are talking to your boss, an older person, or interviewer you use usted.
I originally started taking Spanish because I had to take it in high school, and I grew to love it. Now, I would love to learn it fluently. I have taken a few courses after high school. I think I need more interaction in order to learn it fluently.
I read that the best age to learn a language is very young. When children are young, they soak up information like a sponge. This makes it easier for them to retain information. Therefore, it can become like second nature to them.