If you are using the very first gradient then that is actually foreground -> background, so you can change that simply by changing the foreground and background colours.
Before you do anything make sure you are using a colour image and not a grayscale image - go to Image and then go to colour depth and make sure it is either 24 bit or 32 bit. If it's a greyscale image then you can't use a colour gradient
Feel free to PM me if you have any other PS questions. Once you learn it all it's a breeze. It's just getting going that's the tricky part.