Start small, really small.

Games are massive undertakings, taking large teams of extremely tallented and highly experienced people years to create.

For example, every version of Microsoft flight simulator takes a team of about 50 people 2 years to create and that's possible only by reusing a good part of the code from the previous version.
Doom 3 tooks a similar team about 3 years, Halflife 2 something similar.

Oh yeah, people actually literally work themselves to death on such teams. It's not unheard of for team members to call in sick one day and be found dead on the couch at home a few days later. Extremely long hours with no decent food will do that over time.

I'm somewhat interested in them myself and have set out to slowly, cautiously, work my way towards a chess program.
But before that I'm thinking of simpler board games like tic-tac-toe to explore the principles.
When I have that to my liking I'm going to start on a program that can analyse a chess position and determine whether an entered move is valid or not.

Don't jump in the deep and try to make Doom 4 from scratch, you'll never make it.