Hi @ines-mahmoud,
> ... am trying to add a new widget or develop a new thing in appBuilder , so how can i do it
I would start learning about customizing ArcGIS Web AppBuilder from this site,
ArcGIS Web AppBuilder Developer Edition
It has good documentation and tutorials to get you started.
> ... if i can code with python
You would code using the ArcGIS API for JavaScript
> i have esri online account so i think i don t need to download the AppBuilder esri
You will need to use ArcGIS Web AppBuilder Developer Edition.
Hope this helps,