Hello Kamali,
Hope you are doing well. The answer to your question is very simple. Once you receive your entrance exams results, let us consider JEE Mains for example. After some days of results, the NTA will unlock the website for counseling or seat selection. There you will be asked to login by submitting your login credentials. Once you login to the website, names of colleges will be displayed and besides even the branches will be displayed. Let's say for example, I want to select CS branch in NITK. Then I will search for NITK college out of the list of colleges and I will search for CS branch out of the list of branches in NITK. You need to select seats in proper order as the server checks the option one by one in a order. Don't make mistakes while choosing options. The one which you like the most should be kept in the first position and the one that you feel ok compared to other options should be kept at last. I hope you will understand after reading this answer.

All the best for your future.