What is declaring … how to declare…
Where can I read these basic things …
Hey There !
There’s many things on youtube. Looking around you will find what you need !
This video by Daniel gives a nice, basic explanation of declaring / initialising variables.
Also: If you search processing.org for the word “declare”, you will also find many mentions in the introductory reference materials.
Thank you all 
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I highly recommend Daniel Shiffman videos from 0 to 9 to learn the basics of programming (include declaration of variables and many useful things) in Processing, is how i started and remembered 