I have the Roku app on my Fire 10 HD tablet. Some people may call it the remote app, but the remote is just one section within the app. The Roku app is not available in the Amazon Appstore, for obvious reasons!
To install the app you will first need to install the Google Play Store on your tablet. Instructions are available on the web and vary depending on exactly which Fire Tablet you have.
Alternatively if you do not want to install the whole play store you could just sideload the APK file for the Roku app, Be careful. Make sure that you get the APK from a trusted source. If you have an Android phone you could install the Roku app there and then extract the APK file using the APK Extractor app (from the Play Store). Then transfer that file to your tablet and install it. Again, detailed instructions for this should be available on the web.
(Edited to add that I just noticed your comment about your age. Don't worry. I'm 78.)