Sister, visit this link
http://www.cssforum.com.pk/beginners-guide.php
and explore and read it thoroughly. after that you may ask any question that arises into your mind.
First step is to select your optional subjects considering your educational background, your interest and scoring trend. For this purpose read relevant threads, syllabus and past papers of different subjects.
First do it, then I will guide you further. In between you may ask questions to remove your confusions.