I would really appreciate if sb step forward to explain me the simplest procedure to change the background of a picture.BTW,i donot use photoshop.Thanks is advance!!

there's no other way than to use photoshop to change the background.

take another picture in different background.

simplest way would be
copy the outline of the thing you want to keep, and paste it on top of the background picture.
There are lots of software packages besides Photoshop that you can use to do this.
Gimp is a good free open source option.

The challenge is to make the background and foreground photos to merge seamlessly (shadow, lighting, color etc).

if you want to do a crude and fast job, download paint.net and copy the outline and paste it on the other photo.