I am going to share my experience with regard to subject selection. Please keep a few points in mind while selecting subject
1. Your Personal Aptitude and Interest
You are the one who is going to make the decision, you are the one who has to study and ultimately it is you who is going to appear in exams. Every person has a better sense of His/her own strengths. Never opt a subject which is against your own interest(just in the advice of others). So, make a list of subjects of your choice.

2. Past papers
You should check out at least 5 to 10 years past papers.

3.Books
Find out the recommended books for the subject.Go to a book shop or library, which ever is feasible, then try to read a few pages. If you find the content in the book interesting and appealing to your mind. Then select the subject.

4. Marking trend
Yes, you should look at the marking patterns. But don't just select a subject on the basis of marking. How is it possible that you make an excellent attempt in your paper and examiner is not going to award you good marks?

Lastly, competition is increasing in CSS, every year more and more candidates appear. We have highly qualified and talented persons in CSS. So, choose properly your subjects, select good books, study well, and hit your target.

From where we can find scoring subjects information?

Go to the Subject threads on this forum, displayed on the homepage. There you'd find multiple threads discussing scoring threads...

Best!