You shouldn't have to be told what to practice in order to know what to practice. I don't know how much you've actually played online matches, but I'm sure that if you've played a few, you should be noticing that some things in your playstyle aren't perfect. What you do then is save a replay of that match and watch what you do during said match. If you see something you do that you didn't like -- for example "Oh man, why did I use Noel's 3C without meter? I got punished for doing that!", or "Oh man, why did I drop that combo? I need to make sure I have that down so I don't miss out on big damage!" -- then go into training mode and fix those things you didn't like in your gameplay. Another big misconception I think people make is that they think that they won't be able to know what to fix, but in reality, if you see yourself constantly being punished for something, or that you keep dropping a certain combo, then that's definitely a sign that you have to fix something.
Again, you shouldn't really need to be told what to practice. If you can pinpoint what is causing you to not do well, then it should already be clear what you're going into training mode to fix.