After the class 12th results, DU releases cut off of various colleges in the month of June. You can directly go to the college with your relevant documents in which your percentage arrived after calculation is matching the cut off provided by the college.

Best four subjects (including one language) is taken and calculated to arrive at a percentage.

You are asking to apply for commerce colleges seems you want to apply for commerce course like Bcom(H), Bcom (Prog).{ Only these two are considered as commerce courses}.

You can take admission to above commerce courses by calculating your precentage in best 4 subjects. For example: PCM + English, lets say your best 4 percentage arrives at 95%, but your admission will be granted after deducting 5% for change in stream. Now you should look for colleges offering you admission on 90% and not on 95. This 5 % may vary from college to college but usually it is 5%. ( Criteria might get change in upcoming 2017 admissions, kindly keep yourself updated)

Percentage is the only basis on which colleges will grant you admission, You have to study commerce subjects from basics. You may face problem in accountancy subject and not in others as many of the subjects are from basics.

To get admission in Commerce you need to repeat your 11th & 12th standard.