Welcome to the Tableau Community.
What you're after is referred to in Tableau as a Quick Table Calculation which you can access by clicking on the 'carrot' (the down arrow) of the Value pill
Please see here for further information on this feature :
training video : Intro to Table Calculations
Also, FYI - In future, I suggest you look to post your query in to the main area of the forums, where it is likely to attract more attention. This has been posted in the UK group area, where there is limited activity.
This is the main Community landing page https://community.tableau.com/welcome
Since you're new to the forums, can I suggest you go to the 'Get Started' area and have a read of the recommended reading (Getting Started in the Forums ). The next time you have a question about how to do something in Tableau Desktop, either click on the 'Ask a question about Tableau Desktop' on the left of this page, OR if you're on the previous Community Welcome page, scroll down a bit and select 'Go to Forums' which will take you into the appropriate area to search/ask your question.
Thank you Donna, it worked
Now learning how to sort items by calculated field.
Apologies for posting in a random area of the forum.
No need to apologise Elena, great to hear you're getting sorted. The Tableu online training videos are an excellent resource to help you learn.