A detailed tutorial on how to properly set exposure is probably more than anyone here would want to contribute since it is already provided in detail in the Owner's Manual which presumably you have or can download easily here.
Setting correct exposure in the Manual mode requires adjusting shutter speed, aperture and ISO speed (camera sensitivity) while using the built-in metering to indicate if the exposure is correct. The process is detailed step-by-step on page 155 of the Rebel T6i - EOS 750D Manual. Page 156 adds the basic settings when using flash. For manual mode, settings are outlined on page 157.
If instead of Manual mode you select one of the auto modes (I'd strongly suggest you start with "P" Program AE), page 159 will walk you through using exposure compensation to change the auto-set basic exposure settings to brighten or darken the image. If you are shooting in a room with light walls and metering off one of those walls then an exposure compensation setting of +2 or +3 may be needed to brighten up the image properly.
If after adjusting the exposure for a appropriate brightness range, you still find you have a yellow color cast, then see pages 132 & 133 to understand 'White balance" which can be set automatically, or manually to preset conditions, and can also be adjusted as needed if necessary.
