I would also say you're too young to take Propecia, but then again it truly seems like your mom's side of the family has some chronic balding! If you wait too long to treat hair loss you might have NONE by the time you act! I would advise you to go see your doctor and explain your family's history of balding at a young age.
Now, if you want to know some information on Propecia and its side effects go to www.propecia.com. Listen, you're only 17-18, and one of propecia's side effects is IMPOTENCE! Reduced semen and more...However, this happens on a small number of males taking this pill. Everybody is different, so again check with your doctor and have him explain all the side effects. Dont be shy about it!
It is important you do ALOT of research BEFORE doing something or taking pills. In conclusion, visit your doctor, investigate, and wait until you have gathered enough info to make an informed desicion.
Check with the Better Business Bureau to check the authenticity of some of the websites you visit online. Just Google it dude. Good luck.