Hi Elena

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 :

online help : http://onlinehelp.tableau.com/v10.5/pro/desktop/en-us/help.htm#calculations_tablecalculations_quick.html

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.

Regards

Donna

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.

Donna