All you need to do is find Dark Souls III in your steam library, and then click play. This will start up the game and bring you up to the starting screen.

After you have clicked play on your Dark Souls III game and loaded into the Starting screen, you'll be met with a button that says "press start button". Now this is pretty straight forward, all you do is click that button and it should load up a screen that says "New Game" at the top. Easy as that.

Congrats on officially starting your Dark Souls III experience. I'm glad I could take you with me through this experience. Enjoy endless hours of pain, rage, and suffering. Cheers!