1.- When you have to resolve agendas, who goes first. It is tricky because depends who goes first the prestige resuls could be different.
The resolution is simultaneous (i.e. you add up the results of both agendas together and adjust the VP track accordingly).
2.- How do you deflate Defcon tracks? It seems is only possible with Television and with some cards. I am right?
Whenever you "Command Influence" you can remove cubes from one battleground, which generally deflates the corresponding DEFCON track (in the same way it inflates DEFCON when adding cubes into one battleground).
3.- And last one, when you move from on territory to another, you can only move from one to one only no? E.g: If I move 3 cubes from Turkey to Italy, do i have to increase defcon marked in italy? do i have to decrease also the one marked in Turkey?
You never "move" cubes over the map. You only add/remove cubes to/from a battleground (without any restriction regarding adjacent or other battlegrounds).
The printed rules come with a complete play-through of a game. You may want to read through that in order to have an example on how the game plays.