How to use mirron body?

Hi Amit Kumar!
Greetings of the day.
The answer to your query is mentioned below:

CASE-1:
If you want to mirror in the sketching environment then follow these steps:
1- Draw half sketch and provide proper constraints to your sketch to make it a fully defined sketch.
2- You may draw a centerline to use as a mirror.
3- Use the mirror command and select the required sketch to be the mirror.
4- Then select the centerline.
5- Then press "OK" to complete the mirror command.

CASE-2:
If you want to use the mirror command for a 3d object then follow these commands:
1- Take one offset plane to use as a mirror plan for the future. The distance should be considered as you want to place your mirrored objects to be placed.
2- Go to the mirror command.
3- Select your 3d sketch.
4- Then select the offset plane that you made.
5- Press "OK" to complete the mirror command.

I hope my suggestions will help you to resolve your problem. If you have any further doubts in Fusion 360 or CATIA V5 then you can ping me in my inbox for further discussions.

Thank You
Best Regards
Azhar