Either search the internet or wait 'til Christmas and go to Walmart lol. Don't buy parts for computers from shops, buy from Ebay or something because if you use parts it can turn out to be more expensive than the original price is.
I earned 100 dollars from doing chores at 11 years old, bought my aunts computer which was pretty crappy. I was kind of new to my computer and got a bunch of viruses until I learned more about Internet, PC Games, Virus scan etc.
I give my computer to my sisters and my Mom gives me a much better computer. She bought a new 2006 computer. And here, still using the better computer at 12 years old.