If you want to make money from your work, you may have to work with your parents or another supportive adult. For example, an art teacher could set up Patreon and have a fund raiser at school to help buy art supplies, and help you complete your comic as part of an art project. They might also have art supplies for you to use.
If you don't have any supportive adults in your life who will help you, then you might have to do it oldschool like most of us probably did before any of this internet crowd-funding stuff was even a thing. Mow lawns, babysit, save every dollar bill you get in a birthday card, hoard your lunch money, whatever you gotta do.
When I was 15/16 I started with very cheap traditional materials. Skecthbook paper, pencils, micron pens, and india ink. This all together might cost $30 to get started, and much less if you are thrifty about it. Especially for online, if you just post cleaned up pencil work, then it's fine (my comic is really sketchy and people seem to like it). You can always ink and clean art later if you want to print or publish. I'd draw like 20 pages then ask to use someone's computer and scanner when it became available. Some libraries and schools will let you use their equipment, so as long as it's not NSFW you should be okay.
I got my first personal computer in 1999. I told my family I didn't want anything else except a computer, and so I didn't get Christmas or birthday or any other presents for a year, and I didn't get my first car until 2006, and I had to do favors for everybody whenever they asked, haha... but I had a computer! OMG I removed so many viruses from elderly relatives computers because I was the "computer expert" of the family after that.
I used the same computer for almost ten years, and by the time it was done, it was super outdated and couldn't even run a browser without crashing. By that time I was old enough that I had a job and could get a new one, though. I was (am still am) always asking people if they knew anybody upgrading equipment at their offices if I could buy the old stuff for cheap or just pull it out of the trash. Trolling thrift stores and used electronics stores for stuff maybe nobody realizes it's supposed to be expensive. Got a pretty decent monitor for $20 once. You can also rent stuff at some places.
For me the trouble was always more about transportation, because both my parents worked and couldn't take me to the library or pick me up late from school, or troll around town looking for deals. By the time I got to be 18 I had made friends with a guy who had a car and likes to draw too, so we'd go places like that together. Making friends and working together is really important!
At this age you should also ask your parents about going with you to conventions and looking at the artist alley. If they can see other people making money from their art then maybe they'll be supportive to help you do that, too. When I do Artist Alley, I get lots of parents asking me about how can they help their kids be more successful.
The tax situation depends on country and financial situation. Like in the US, if your parents included in their income (because you are a dependent of theirs), then they'd have to pay income tax on it, but most people who are paid by regular paychecks already receive tax refund from their withholdings, and what you owe would simply be deducted from that. Unless your parents are small business owners or you're making thousands of dollars a month, it is probably not an issue.
Anyway, good luck! I wish all this stuff existed online when I was as young as you are now.
