Lisa

Go to the customization screens for the entity

Administration -> Customisation -> Opportunity

Note: Address is a secondary entity so it will be in the drop down list at the bottom on the Administration -> Customisation page.

On the fields tab, look down and find the field you want. Then look to the right of the screen and you'll see a little edit symbol under the column 'Field Security'. Click than and you will see a check box that can make the field required.

I don't have any customization choice. 

I don't have any customization choice.