what is the basic idea behind steganography?ie ,how do you get the hidden information? suppose if it is an image and some text is within that image...how do you get that text?..
Every stenography algorithm is different in that respect. Every algorithm hides the information differently and thus getting the information back is different.
A simple example goes like this - Each pixel of the image is composed of 3 bytes, one for red, green and blue. Most people can't detect a difference of one bit in the color in an image so one option is to use the least significant bit of each color channel for your data. This way you can store 3 bits of information in every pixel with very little effect on the general quality of the image.
To get the information back you'll need to read the first bit of every color channel of every pixel and gather all the bits together.
This is just a very simple and almost trivial way to do stenography. Real stenography algorithms are somewhat more involved. Like in cryptography, there is no way to generically "unhide" all stenography. you need to know which algorithm you're trying to decode.