I am not a doctor, but have had some results using minoxidil and Foligain’s Minox&Triox spray. As was pointed out in the other comment, hair follicles will start to die as you age (assuming you’re male) this is caused by DHT which is a result of testosterone breaking down. First the follicle goes dormant, and then dies. Depending on how advanced your thinning is, you can likely reactivate and save between few and most of the follicles. I hate recommending Reddit, but r/tressless is a good resource, but they pass around a lot of lingo so be prepared for that.