Hi Iven,
It's good that you are here. If you are noticing your top thinning at 23 there is a good chance you will end up losing a lot of hair down the line (sorry about that). Really the BEST thing you can do right now is start taking propecia. It will slow your loss almost to halt. It will give you years longer with your hair. As for side effects, as you know all drugs have possible side effects. It would be worth your while to give it a try and see if it works for you. The chances are that you will not have any negative reaction to it and you will keep your hair.
In my (and most peoples) opinion you are far too young to be getting a hair transplant. There is just no way to know how much you will lose in the future.
About propecia and a HT. Yes it is advised to be on the medication for the rest of your life, or basically you will lose all the hair around the transplanted hairs and look very strange.
So my advise to you is:
1. Go to your doctor and ask to try propecia.
2. Spend a good amount of time on this forum reading threads so until you really understand what a HT can do and what it can't.
3. Wait a while before considering a HT. Do not rush this.
Welcome to the forum and keep asking questions.
Good luck, M.