Hey Swati,

There will be an only registration for Delhi University. If you wish the study there please keep yourself updated with the site du.ac.in

During the registration process you'll be asked for various necessary details and the courses you wish to select as well. Once the registration is done, you'll be asked to pay the registration fees and you're all set for the counseling round.

During the counseling round, Delhi University will keep posting cut offs based on the overall result and number of applicants. Since, you want to take B.Sc Botany, you have to check it's cut-off for respective college. If your marks are above the cut off then immediately rush for the form submission in the respective college. 

PS: For all the B.Sc Programmes they take your three main subjects' marks i.e Physics, Chemistry and Biology in your case. 

Hope this helps, good luck!