Do the usage of hair straitener afffect on hair..? I feel that my hair is becoming white due to use of it. I'm just 21.

What type of hair straightener? Are you talking about hot irons or straightening solutions (basically perm solutions)? Yes, perm/straightening solutions will leach color from the hair, but this is not permanent. When the hair grows out, the color will still be there.

Is your hair already blonde? Heat will cause you to loose color more quickly, try to limit the use of your flat iron to 2 or 3 times a week, and turn the temperature down.

Absolutley. After I started using a flat iron on my hair, it started to get brittle and platinum blonde.
The damage was so extensive (not just from the straightening, but from the constant highlights) that I had to chop my hair off and dye it brown.
I hate it, but it was my only option.