Decide If You're Upgrading or Doing a Clean Install
You can upgrade to macOS High Sierra which keeps all your apps and data on your machine. Or, you can do a clean install of macOS High Sierra which gives you a fresh start.
If you have a newer Mac or you did a clean install of macOS Sierra last year, you can probably go ahead and perform a straight upgrade that keeps all your data on your Mac.
If you have many problems with your Mac and the last time you did a clean install was years ago, you may want to go this route. It wipes your Mac and gives you a fresh start. When you do this and then setup your Mac as a new Mac, you can solve many lingering problems. It's more involved, but can be useful for people who are having problems before the upgrade.