What’s the most appropriate Metaphor for submitting a selected Item in android App?

I have designed and implemented an android application for patients aged >30. While conducting a usability Test, I found the following Issues :

1. CheckBox: When the user searches for particular item and decides to choose the one he desires, the checkbox is hard to be activated on small-devices.
2. Most of the participants on the test, tend to have difficulty finding a submitting button which is at the top-right of the tab-menu. What icon should I use or any other method/button? would hover button solve the issue or should I use animation since I am leveraging material design.