The best thing to do is to work for the money to buy one yourself. Gaming PC's aren't the cheapest of toys to ask for.
If you are going to be 15 soon, some states--assuming you are in the U.S.--allow you to work under special conditions if you get a worker's permit. You just have to find a place that will hire you. Fast-food places would be your best option, but you can always ask around and ask your parents if they know some place you can work to earn money and buy something. Worker's permit minors aren't allowed to work over a set amount of hours and are only allowed to work during non-school hours so I think your parents will be okay with it.
There is also the option of asking for a bigger allowance for doing extra chores around the house, or doing chores for neighbors. I used to help my neighbor with their food truck business for a small pay. Obviously, it takes time to raise the money to but a gaming PC, but it's never too early to learn that you can't always just ask for something you want and get it. You need to work for what you want.
If you are too lazy to work for it yourself, you can always ask mommy and daddy nicely.
