If you can, find a topic you're passionate about and explore it. This tends to lend itself to "passion projects," advanced classes in a certain field, community service, etc. The most important part about this, though, is that you enjoy what you work on.
Abide by the "Law of Two Feet:" If you do not like an extracurricular after giving it a fair try, walk away. There is no shame in quitting something that a parent or advisor told you to do if you are not enjoying it!
Your teachers want to see you succeed. If you are struggling in a class, talk to the instructor. They can often point you in the direction of extra resources, "office hours," or free peer tutoring to help you understand the material!
Don't overwhelm yourself with super hard classes, a billion activities, etc. With that being said, though, if someone warns you that an academic decision (like dropping a class) may not be a good idea, check in with yourself before following that advice. Even though advice is usually helpful, you know yourself best; trust your gut.
Have an idea of what you want in a college by the end of your junior year -- this will make choosing which schools to apply to easier. See the "Finding Schools" tab for more!
You do not need to apply to 20 schools! I would recommend choosing around 2 "safety schools," 2 or 3 "match schools," and (if you want) a few "reach schools." When I applied to colleges, I decided I would rather apply to relatively few schools and focus my attention on scholarships!
Ask for letters of recommendation early on (I'd suggest doing so by the end of junior year). This will give recommenders ample time to customize a letter (instead of using a generic note for all of their students). If a teacher is swamped with letters or does not know your well, ask someone else (perhaps a boss at work, sports coach, or religious leader) who knows you better!
"Demonstrated interest" is, essentially, when you show a school you're interested in them by interacting with them. This is shown through in many ways, from attending admission officer visits in your area to viewing their websites. The "Resources" tab has a list of schools that track demonstrated interest!