My keys won't type on my new iMac so I can't get in.

try Tom Gewecke's solution first. if that is not it, is the keyboard the rechargeable apple Magic Keyboard? if it is, you can try using the charging cable to connect it to the Mac to sign in. then try and disconnect the keyboard from the bluetooth and then reconnect it.

Cksquared wrote:

In a new iMac some letters won’t type

If it's 789uiojkl, try hitting the option key 5 times. Once into the Mac, make sure Mouse Keys is turned off in Accessibility preferences,

Try using a different keyboard to see if the problem is keyboard or computer related