Building a Gaming Computer

I'm building a gaming computer but I have absolutely no idea where to start. I already know someone who can put the computer together but I don't know what my processor, motherboard, RAM, etc. Should be. Can someone please list parts that I should buy? My ideal computer would be able to run games such as minecraft, and different steam games on high quality settings. My budget is at about $800 - $900. Please please please help me!