The Mode Palette is under the Image file menu in Photoshop.
In this screen grab you can see that the mode is RGB and we are working with 8 bits per channel – i.e. levels 0 to 255 where 0 is no colour and 255 is maximum colour. So 255 in the red channel means maximum red saturation for the colour space – see this short explanation of how computers define colour

These two settings – RGB Color and 8 Bits Channel are the ones you need to select. Don’t worry about the others for the time being, just check to make sure these options are ticked.
{ Just to tickle your appetite, maximum saturation varies depending upon the “flavour” of RGB. Red255 will be more saturated in adobeRGB (a wider colour space) than it is in sRGB – that’s why the colour profile attached to the image file is so important – we’ll deal with that later ! )