At 18 you should have good effects from HRT.
A couple of points, once you are on HRT there are irreversible effects. After a while you will be sterile, once you develop breasts they don't go away without surgery. You may lose the ability to have an erection, this seems to be quite variable as our brain is our biggest sex organ. I haven't had a spontaneous erection in at least 12 months. So it isn't something to be taken on lightly, as in I think I'll just try it, I can always stop.
When you come out to family and friends and work that you are TG and start living FT as female, there is no real way to go back. I'm Cindy and people knew my 'male' person before hand. They accept me as Cindy, but there is no way I can suddenly go back to being 'him' again, even if I wanted to, I would be rejected, at least I think I would.
HRT doesn't stop facial hair growth, it may slow it, the only way to get rid of the facial hair is by laser and electrolysis. Laser is cheaper and quicker but only works on dark hair, skin type is also important. Pigmented skin is a bit of a no no for laser as it can destroy melanin dark cells, effectively burning you, so dark skinned African-American girls in particular need to seek very good professional advice. If you have blonde or grey hair electrolysis is the only way at the moment.
In general forget the 'home' systems, they are no were near powerful enough for most of us. Some of the Asian girls have had success with them but they tend to have sparse dark hair.
The sorts of things you need to think about is how are you going to live your life? Will work accept you? Can you financially afford to do this? Can you emotionally afford this?
There is no going back, is that OK with you? You may lose your family, your friends, your religion people may reject you (seems to be a USA specific problem).
These are some of the things a therapist will help with.
Once you are on HRT you need advice on; depression, as T levels drop you may become depressed, but you probably are anyway, Headaches (migraines) and nausea may develop, mood swings, liver dysfunction, cardiac problems and increased risk of breast cancer can occur. Most of the indications are small. But they are there.
Therapists can help with all of this.
Personally it was a no brainer. The side affects meant nothing to me, I used to have morning sickness, but I almost enjoyed it

Now everything has settled down.
In Australia the standard of care is written down legally, we have to see a psychiatrist, at a point in time we have to be seen by an independent psychiatrist, our case is discussed, we can then be recommended to a endocrinologist, who is registered to look after TG people. We the have to live RLE for at least 12 months, our case is then reassessed and then we can be recommended for surgery.
Sounds a run around. In my case I saw my primary psychiatrist 4 times, I saw the backup once, they discussed my case in the corridor. I was given the letter, and saw the endo the following week and was on HRT that day.
I now see my primary psychiatrist about every two to four months, which is sometimes just a phone call to make sure I'm OK.
My family Dr looks after my scripts and I have blood test every few months.
One thing about all of this 'over the top' care, is that I feel very safe. I know people care about me, and that I an contact with people if I feel down or upset. I haven't, but it is there if I need to.
So.
Sorry for the long post. But what I was trying to say, is that a therapist and the right channels does have good advantages, in my opinion.
Cindy