You'll not become truly good unless you build your own teams. Starting with sample teams is fine, but the building process helps you understand gameplanning better, and allows you to better suit your team to your strengths.
It takes practice and experience to be good. Start off by watching ladder games of whatever meta you are trying to get into. When you're comfortable with that, move onto high level tourneys. There are plenty of youtube channels that can show you either of these, be it Smogon or VGC.
One thing I'd like to note, since other posts mention otherwise, is that prediction isn't that big of a deal. New players tend to get stuck in the mindset that whoever makes more predictions is the better player. This isn't the case. You need to assess your opponent's options and play according to the gameplan you set from team preview. You can make predictions along the way to help you, and you'll get good at this with experience, but don't think the game is about predicting correctly every turn when that isn't needed.