Need help choosing parts for my pc (I don't know much about computers)

Hi, and thanks for clicking this. I currently have a laptop I bought 2 years ago for, but I'd like to upgrade to a desktop since I don't move it much anymore since I have an iPad and bring that around with me instead. It's been serving me well so far with what I've used it for, mostly school and games (league of legends and I plan on playing guild wars 2). I'm not familiar with computer parts so I'm not sure how to determine which video card, processor etc. I will need. I'd like to spend 600 but I can spend 800 if it's really needed. I also have a Sony vaio tower from about 8 years ago. Can I use any parts from it, like the tower, cd drives, and hard drive, it has no power supply since I gave it to my aunt since hers broke. I'd like to build it in 3-4 months so I can shop around or wait for sales. I also have a windows 7 os disc, keyboard, mice, and a 24 inch tv I use for a monitor. I live in mississauga, Ontario so I plan to shop. At tigerdirect.ca or newegg.ca. Thanks for reading and any help.