I would google if you have any lgbt friendly drs office that offer help with HRT and if not then planned parenthoods are also a good place to start. Before considering starting you make sure you look into all of the changes and possible side affects that may come with taking testosterone so you're absolutely sure and aware you'd be comfortable with those changes. There are multiple options you have for taking it like gel, patches or injections but gel and patches can sometimes be a little complicated with insurance and injections are the cheapest option I think

The most important place to start is finding out if you have or can get medical insurance. That can take some time. Go through your insurance when finding providers to ensure they will help to cover costs. 18 is a good time to switch from a pediatric GP to a new primary care doctor. You can start by seeing a general doctor, explain your struggles with gender dysphoria and asking them if they can refer you for gender affirming care.

The answer to this heavily depends on where you're located as it's different in every country

I saw that someone mentioned medical insurance, so idk if this will help but Starbucks (at least in the US, i think- may be applicable in other areas) offers medical insurance for gender affirming care! I will also be starting testosterone soon, so I understand what you’re dealing with, good luck!!!!

Different places have different rules, but for me I had to go to therapy for 2 years to make sure I wasn't just kidding

If you're in the U.S, without insurance, check with your local Planned Parenthood. Mine does informed consent HRT and they might for you, too