I have imported one CSV file where I have zones of the city. Is it possible to get one big polygon of the city?
You have the
dissolve indicated by erik, tool which you probably used now, it offer you to dissolve with options.
If you want to merge selected polygons you can also use the
Merge Features in advanced digitizing, you don't have one big polygons, as
dissolve tools, but it could be useful depending on what are you looking for.
More over there is a extension :
dissolve with stat but actually it's not update, works with older version of qgis.