I am not a developer but need help to create a formula that will allow me to do the following

1. I have a custom field in 'Opportunities' layout called 'Proposer' that has a drop down box with about 12 choices. It is a mandatory field in opportunity creation at the moment.

2. I also have a similar custom field in the 'contact' layout with the same drop-down values.

3. I want to create a formula that automatically imports the value from the custom field in the 'contact' layout into the custom field in the 'opportunity' layout, coming from the contact that the opportunity relates to.

Can anyone help me? I am based in London, UK.