Hair Regrower

So: I'm not saying I'm balding yet, but I do think I can spot thin hair when I look in the mirror at some places, so I'm thinking it might be reasonable to do something against it already. A guy I know recommended me hair regrower, saying it really works. And no: I'm not going to ask you which you use, or could recommend me (because it's unlikely my country will sell those exact ones).

I'd rather have the following questions answered:

-What should I watch out for when choosing what hair regrower to buy?
-How do I apply it correctly?
-How long will I have to wait to see improvement?
-Anything else I should know?

I will appreciate every help I am given :)