I fairly new to this computer building. Though I need help getting started.?

Im fairly new, i been on youtube and seen alot of videos about building a PC. None of which have scared me away from trying. It seems very simple. I learnd some terms too. I been around computers all my life since i was 3. I always figited with electronics such as a cell phone i tried to open 3 months ago. Which i shouldnt of done because to my dismay when I opend the cell phone screen it leaked some chemical into my finger that burnd real bad for 3 hours. then felt warm for 3 days. Anyways, I need to know what components to build a ultimate gaming PC. I do wish to build computers in the future and sell them privately. I couldnt believe my eyes when I seen 15 year old boys build computers on youtube. I was amazed! Please tell me, what is the best motherboard, videocard, Hardrive case. And anything else nessasary to build a ultimate gaming PC. Also please answer this " When i sell my PC's, can i pre install games to get customers interested?"