I would suggest checking out newegg's 3 part guide on building a computer on youtube (I used this on my first build too). I am unsure if you need Os and depending on your budget, you can get a better cpu/gpu.
$800 -
PCPartPicker part list:
http://pcpartpicker.com/p/3HGXk
Price breakdown by merchant: http://pcpartpicker.com/p/3HGXk/by_merchant/
Benchmarks: http://pcpartpicker.com/p/3HGXk/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($109.99 @ NCIX US)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($27.99 @ NCIX US)
Motherboard: Gigabyte GA-970A-UD3 ATX AM3+ Motherboard ($79.99 @ NCIX US)
Memory: Kingston 8GB (2 x 4GB) DDR3-1866 Memory ($69.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($56.98 @ OutletPC)
Video Card: EVGA GeForce GTX 760 2GB Dual Superclocked ACX Video Card ($239.99 @ Newegg)
Case: Corsair 200R ATX Mid Tower Case ($44.99 @ NCIX US)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($47.99 @ NCIX US)
Monitor: AOC E2360SD 60Hz 23.0" Monitor ($129.99 @ Amazon)
Total: $798.90
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-10 22:15 EDT-0400)