Packages are published to pub.dev. You can search for any packages in that website and you can find a lot of information about the package.
To install a package open pubspec.yaml file and under dependencies add the package name with the version. For example, to install SQFLite in flutter you need to add the line below the dependencies attribute:
dependencies: ... sqflite: ^1.3.0
The line sqflite: ^1.3.0 specifies that we are going to install sqflite and the version is 1.3.0.
After that run the following code from the project directory:
flutter pub get
This will take some time and will install automatically.
To check if the package has been successfully installed in your project, open pubspec.lock file and check if the package has been listed in that file.
When running flutter pub get for the first time after adding a package, Flutter saves the concrete package version found in the pubspec.lock lockfile. This ensures that you get the same version again if you, or another developer on your team, run flutter pub get.