First off, congratulations on getting to this point! Making the decision to start living as a guy is scary but really worth it. I'd say the first thing is to start dressing in a man-presenting way and going out to places like that. To school, to the mall (do people still go to malls?), to your job if you're able to get one. Even though you're not on HRT there's still a pretty good chance you could pass as a boy just from having short hair, jeans, and a t-shirt. When I was a teenager I would go to school with my hair tucked into a hat so it looked really short, and soak up all the gender euphoria when people would mistake me for a boy. I would also "borrow" (aka steal) a lot of clothes from my older brother, but if you don't have that option you can always sneakily go to the men's section of a Target or something like that just to get some boy's clothes. If you're worried about your chest and can't get a binder, I highly recommend this Nike sports bra. It was my lifesaver before I was able to get a binder. Just walking around in outfits that make you look and feel like a guy is a great place to start with experimenting, even if you don't officially come out to anyone or anything. It'll help you get a feel for what sort of presentation feels good to you and also see how people react to your masculine presentation.
Some other assorted things:
- put down a boy's name for yourself at starbucks or other cafés so when they call out your name they assume you're a boy
- if you have friends you can trust with gender stuff, ask them to use he/him pronouns for you to try it out
- go to meetings for LGBTQ+ support groups in your area dressed as a boy and introduce yourself with he/him pronouns if they ask
- you're not going to pass 100% of the time; you're probably gonna deal with disappointment when you think you look super manly, and then get misgendered, and it's gonna suck. when this happens, just remember that the way you feel about yourself is more important than other people's assumptions about you, and just because they assumed incorrectly doesn't mean you did anything wrong.
Anyways, from one trans guy to another I'm happy for you. Good luck dude!!