I haven't been on T very long, but I can answer some of your questions.
Typically I think you have to be 18 to get T, but if your not done with puberty you maybe able to get on blockers with parental consent. I don't know about starting T before growth plate closure because T can be converted to E and E is supposedly what causes growth plates to close. Typically female bodied people stop growing 2 years after their 1st period.
You have to find an endo, other doctor, or gender center that will do it. I think plan parenthood does it, but I'm not sure. I recommend utilizing state trans* resources. Informed consent is easiest, but therapy is beneficial and some people require a letter.
You can take it in patches, gels, creams, and injections. Injections are the cheapest and patches are the most expensive in most cases.
Here is a better description here
http://www.ftmguide.org/ttherapybasics.html and
http://www.ftmguide.org/ttypes.html and
http://www.ftmguide.org/tandhealth.htmlPositives/neutral
It causes body hair to masculinize: face, arms, legs, chest, and hairline.
Deepening of voice
Emotional Changes for better or worse
Clitoral growth
Stops menstruation
Changes fat distribution to a more male pattern, but cannot change bone structure.
increased sex drive
easier to build muscle
Negatives
Acne
enlarged liver/liver damage/ liver cancer (men have higher risks of these conditions)
Heart health
Balding
Stronger smelling BO