Ok Bob... first... check under the IMAGE/Mode menu and make sure you're working in the RGB Mode.
Second... You can't ADD type to an existing layer. Just pick the Type tool, click in your document where you want the type to be, and PS will automatically create anew special 'type layer' for your text.
Afterward, should you care to, you can "Rasterize/Render" your type layer(s). This will make them no longer editing using the Type tool, but completely editable using any other command/function/filter in Photoshop.
That help?
PS: on a PC, press the "Print Screen" button at the top-right side of your keyboard when you want to 'capture' the screen contents. This copies the screen image into the Windows 'clipboard' (
a temporary 'virtual' storage space). Then while in Photoshop, press Ctrl/Cmd+N to create a New Document. Accept the default settings except for the colour mode; make sure that's on RGB... then click OK. When the new doc opens, press Ctrl+V to paste the captured screen image into the new document.
Let me know if this doesn't work right for you. As soon as you do this once, you'll understand how simple it actually is.