Many suggestions put forward but unfortunately many are off the mark, except for ortega's.
1) Use a vector based program - InDesign, Illustrator, Freehand. Take your pick.
Reason - you want your text to be sharp and crisp. Also your text can be scaled up to whatever size you need.
2) When setting up the document size, you can work at A2 or A3 (better/faster) and allow for a bleed area of at least 5 mm (so you don't get white edges when trimmed to size). The print service provider will be able to scale up when the file is sent to rip.
3) Import your graphics in tiff, not jpeg, 300dpi is ideal but if you're gonna print at A0, expect some pixelization.
4) When you save, you can either save in the native file format of the vector program (very big and there are inherent risks as you need to ensure the links to the graphic files are maintained and fonts used are included. You could also convert and save as a Postcript file (eps format) but the file size is likely to be very big.
Best solution is to save as high resolution pdf for press quality. Graphics, text and layout flattened and converted. Less chance of font issues or graphic issues (rgb to cmyk conversion). File can be previewed and printed easily.
5) Don't ever use MS Word. Very poor layout, font and graphics handling. Whoever suggested this ought to go and read up more. Don't ever save as jpeg if you want the best quality.