How to Use Formula field in sales force

Follow these steps to navigate to the formula editor:

1. From Setup, open the Object Manager and click Opportunity.
2. In the left sidebar, click Fields & Relationships.
3. Click New.
4. Select Formula and click Next.
5. In Field Label, type My Formula Field . ...
6. Select the type of data you expect your formula to return. ...
7. Click Next.

For more information about working with formula fields please check with below links which might help you accelerate further with the above requirement.

• https://trailhead.salesforce.com/en/modules/point_click_business_logic/units/formula_fields
• https://help.salesforce.com/articleView?id=customize_formulas.htm&type=0

Hope this helps.