i need to customize a new coordinate system in arc map 10x and where i can save it ?
click the Data Frame Properties -->Coordinate System-->Add Coordinate System-->New. Than choose projected or geographic and define the parameters. After closing the dialog click right the on the name of the new coordinate system and add it to favorites. A PRJ is created in your profile under C:\Users\<your name>\AppData\Roaming\ESRI\Desktop10.2\ArcMap\Coordinate Systems.
I'vemoved your post into the GIS space. You will get a much better answer here as the GeoNet Help is intended for community help and feedback. I have also added in a few additional tags. You can see moreon the community structure, and what topics are under each space from the following documents:
GeoNet Community Structure
ArcGIS Discussion Forums Migration Strategy
Retrieving data ...