I've implemented an application for Ubuntu Touch, using QML. How can I create a "package"(something like APK in Android) for the app?
You create a "click package" from the Ubuntu SDK.
Have a look at "Packaging click apps": http://developer.ubuntu.com/publish/apps/packaging-click-apps/
"apps are shipped as click packages. To generate click packages, the first step is to open your project in the Ubuntu SDK and click on the Packaging tab on the left hand side and fill out the form presented to you."
Once the package is created, you may also publish/upload it.
You can find other relevant documentation on the Ubuntu Developer Portal: http://developer.ubuntu.com/