Don't overthink it. Continue to ride your uncle's bike as long as he'll let you. Don't compare yourself to the serious athletes on this forum and think you need to reach their level to "do it right". Anything is better than nothing. Forget about the heart rate monitor, it's not necessary for riding. If you are still riding in a year then repeat for the following year.
edit: revision. Do things that help motivate you to continue riding. If tracking heart rate or distance help motivate you then do that. Don't compare yourself with others. Learn how to do regular preventive maintenance on your bike - clean and lube the chain, etc.. Learn how to fix a flat and be able to do that while out biking. Good luck, report back with progress.
Several $59.95 heart rate monitor straps here:
https://www.pcmag.com/picks/the-best...-rate-monitors