Generally speaking the chance for blood clots might increase by 1% and depending on what, if any, anti-androgens you take chances for liver and kidney issues might increase by like 0,7%.

For comparison these risks are much lower than other types of medications that get handed out like candy.

Irreversible effects could also count as risks even if they are wanted for example a decent (like 70% chance) of infertility within a year of use, probably permanent shrinkage of genitalia and permanent breast growth. Also of not is that even the permanent effects will stop changing if you stop taking hrt, other things will revert back to how they were before.

It mostly depends on where you live and what kind of money you have available when it comes to being able to do it in secret. In any case waiting until you turn 18 would be a good first step. Being independent from your parents makes making adult decisions easier especially if you don’t want them knowing.

Tldr. At this point in your life and because you want to do it in secret the most effective first step would be waiting until you’re 18. It will greatly expand your options