What you are looking for is called Reshaping Text using an Envelope Warp in Illustrator. This will make "one word text go from big to small all in the same straight line."
Go to the tool bar and click Object > Envelope Distort > Make with Warp> The Warp Options dialogue box will show settings and a preview box to use.
To get the look you want try the rise>horizontal radio button> Zero Bend>then go to the Distortion slider and try for a negative slide around -50 or more. Do not use the Vertical slider.
Hope this will help with your education!