Generally, it is considered that the earlier you can start (either by puberty blockers or hormones dependent on age), the better.
However, this is not always possible for a variety of reasons. The most important question I would ask is: Do you have a supportive family? If your family is supportive, then I would start talking to them about how seriously you feel about this.
Even if they are supportive, they may have concerns about starting HRT before you are a legal adult. If they do, I would see about whether they're willing to go with you to a gender identity therapist who can evaluate you and maybe help explain to your parents.
Puberty blockers are less "threatening" since they simply put off puberty and biological changes, and if you have a supportive family, they may be willing to go with this option. I suggest this because even if you have already started puberty, the "everyone knows" "knowledge" that puberty is between 13-18 is incorrect. Physical body changes continue through your 20s. It's generally thought to extend until, iirc, roughly 23 for those assigned female at birth, and up through 25-27 for those assigned male at birth.
I do also want to mention that AMAB folks can experience a "growth spurt" around age 25 that causes the body shape to become more masculine, with broader shoulders, etc. I've known quite a few AMAB folks (regardless of identification) who have talked about having extremely slender bodies, not terribly broad shoulders, etc, at age 20, and then around roughly 25, their body changed to become more masculine. This isn't to say it's guaranteed, because I do know plenty of AMAB folks who remained with a slender body, but I think that is in part genetic.
If you do NOT have a supportive family, then you are likely going to need to wait until you are independent. If you are in the US, you should remain on your family's insurance until age 26 (I believe they are required by law to cover you, but you may want to double-check that), and you will want to check out what they cover. Therapy is usually covered, and HRT is covered for a lot of folks I know, but surgeries often are not. But, once you are an adult and on your own, you would be able to take advantage of the insurance coverage to pursue transition regardless of your family.
Any other advice I would have to give really does depend on the answer regarding your family, and also life-plans. For instance, are you planning on going to college? If you are, you will likely want to check out how supportive they are of trans* folks. Even though a lot of colleges these days have LGBT clubs and resources, that doesn't mean they are trans friendly. There are lots that are just LGB (or even just LG, and to hell with the other letters) friendly and don't support trans folk at all.
Anyway. I look forward to hearing from you, and welcome from another newbie. *hugs and cookies of choice offered*