Salesforce trigger to update a field based on a picklist value on a visualforce page

Salesforce trigger to update a field based on a picklist value on a
visualforce page

I'm trying to find a way to have a trigger update a picklist field based
on a picklist selection from another field. I want the trigger to update
on the insert of the record.
For example, I have a field called Address__c that is a picklist field and
I want the trigger to select the appropriate location based on the address
selected.
Address_c 123 Main Street selected, updates Location_c "Main St."
Address_c 321 Elm Street selected, updates Location_c "Elm St"
I've done an extensive search on this topic but haven't found anything
that will update a picklist field from a picklist. Any suggestions?