I made a page but didn't make separate file for CSS. can you tell me whether its necessary to make a separate CSS file or not. up till now i didn't face any kinda trouble.

It depends

how mr chris?

If you use more than one page it will save you a lot of work.
Because you will only have to include the css file with an include link.
link: <link rel="stylesheet" type="text/css" href="file.css" />

The big advantage about seperate css is that if for example you want all the text from the site to turn blue you only have to change the css.
If you would'nt have the seperate css you would have to change every single page.

Pieter hit the nail on the head - having your styles embedded in the page defeats the advantage of being able to make site-wide style changes thru the single css file.