Your BEST option would be to build one. We'll give you a list of parts, and a very good tutorial on how to put them together. All you have to do is order them and follow the tutorial, and you should be fine. I built one just after my 15th birthday.
Your next best option would be to get a regular desktop computer. All-in-ones are actually quite a ripoff most of the time, and if you just get a regular desktop and buy the monitor separately, you'll be getting a much better computer for a much lower price. Just tell us the MOST MONEY YOU CAN SPEND, and LET US KNOW IF YOU CAN BUILD IT, and finally EVERYTHING YOU INTEND TO DO WITH THE COMPUTER and we can give you something good from there. Also, never, ever, ever buy anything from Dell, HP, Alienware, Acer, or any company that is owned by those companies. The only brands we approve of are Sager (which are laptops only), ASUS, Lenovo and MSi. Almost always, the other brands are crap. But we always encourage people to build a computer, because it really is the best option.