Setting your default signature in Outlook
STEP 1:
Open Outlook. Go to Outlook > Preferences menu.

STEP 2:
Click on Signatures.

STEP 3:
Click on Default Signatures...

STEP 4:
Click on the menu list under the Default Signature column next to your account and select the signature you wish to use for that account.

STEP 5:
Click the OK button when you're done and close all windows.