Printer prints same goobly gook over and over how do i clear

Solution: Perform the print head cleaning procedure MP750 / MP780

Print Head Cleaning / Head Refreshing If the printed Nozzle Check Pattern is broken, missing, or a specific color is not printed, clean the print head.

1. Press the <Menu> button on the printers operation panel repeatedly until [MAINTENANCE] appears in the LCD.
   
   
2. Press the left [<] or right [>] arrow buttons to select the cleaning setting you want. Select [HEAD CLEANING] or [HEAD REFRESHING] for deep print head cleaning.
   
   Note: [HEAD REFRESHING] uses more ink than [HEAD CLEANING], so perform [HEAD REFRESHING] only when necessary.
   
   
3. Press the <OK> button.
   
   
4. Print a Nozzle Check Pattern and perform at least two Head Refreshings if the pattern is not correct.
   

Now if you mean not printing the letters correctly go to this site and download the drivers and reinstall them. Also you will find manuals on HOW TO sort of thing:
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=SupportDetailTabAct&fcategoryid=238&modelid=10444#DownloadDetailAct

hope this helped, please leave rating