You need to wear them consistently. This isn't like popping on a retainer at night and forgetting about it until morning. To get the results you want, you have to wear your aligners for 20-22 hours a day. That means taking them out only for eating and drinking (and maybe a quick selfie).
You need to be disciplined about when you change them. With Invisalign, you switch to the next set of aligners every two weeks or so. But if you don't follow this schedule, your treatment could take longer than it should.
You might still need traditional braces for some problems. Invisalign can handle most teeth-straightening issues, but not all. If you have severe bite problems or serious tooth misalignments, you might still need traditional braces to get the results you want.
It's not cheap, but it's worth it. Invisalign isn't cheap. But if you factor in the cost of missed work days due to pain and discomfort from metal braces, plus the potential for long-term dental problems caused by those same metal braces, Invisalign can actually be a pretty good deal.
You need to see a dentist who's trained in Invisalign. Not all dentists are qualified to use Invisalign. They have to go through special training and get certified by the makers of Invisalign. So if you want Invisalign, make sure your dentist is one of those specially trained folks.
