Scroll all the way down to the bottom. There's a check box "Don't Show Welcome Screen Again"
You can relaunch the screen at a later date by selecting Help>Welcome Screen...
If you are a trial user, there isn't a way to easily disable the Welcome screen. That's because we hope you'll use it to better learn how Photoshop works during your trial period. If you do close it manually, it won't appear again until you relaunch.