When I type on my wireless keyboard any letter a type of code comes up, for example the letter D comes up as cde3 and the letter g as gt5 .No letter works only the space bar ?

I agree with den.thed that the keyboard is bad. If you notice, the characters you highlighted are grouped together in columns. Is there any chance that someone spilled liquid onto the keyboard? Sugary liquids are usually very easy to detect because they leave sticky residue no matter how much you try to clean them up, but water and other non-sugar beverages can still cause issues but be much harder to detect.

The keyboard is bad.

Replace it with a new Apple keyboard or optionally, you can use most any generic USB and bluetooth keyboards with your iMac.