Aggregate of  Physics,Chemistry,Math/Bio/Biotech and also english of your class 12 marks. 

greetings,

you should have studied science stream to get admission in bsc hons chem. best of four percentage is calculated i.e best four marks/4.

english is compulsory in calculating best of four.it has to be included and rest 3 best eg chem+phy+maths or chem+phy+bio

if you didnt not have science stream -2.5 will be deducted.

following is the previous years cut off:

http://www.du.ac.in/cut-off.html

following is the course/syllabus for bsc hons chemistry in delhi university:

http://www.du.ac.in/du/uploads/old-ug-courses/04082010_chem.pdf

hope this helps

good luck!