When is the right time to take admissions

For degree colleges, the classes will start from June 1st, most probably.
You decide on the college in which you wanted to do your B.Com. Select 2 or 3 good colleges. Go there and ask them about the applications for admissions. They will give you if already the admission process is started. Otherwise, they will tell you when they will be available. Where they will be available and any application fee required. You take the application and apply even now before getting the results. If time is where you can wait for the results/ Otherwise you can apply. Anyhow the admissions will be after the declaration of results of intermediate only. If you apply now you have to submit your copy of the marks lists after you get the result.
There will not be any admission tests for many colleges for BA, B.Com and B.Sc. They will give you seat based on your intermediate marks.

There are many career options other than engineering after completing class XII with science stream.

One of the most important factor which is to be seen at this juncture is what is your interest area. Have you more interest in a particular stream like commerce, medical, pharmacy, humanities or law or things like that. Until unless you find that you will not be working hard in an area because it is not your cup of tea, you do not chart out a successful career aspiration.

Anyway, whatever be your interest area, you have to identify it and then pursue it rigorously with hard work and persistent efforts. That is the only recipe for success.

After completing your XII, you can get admission in B. Com. in any college or university based on your marks of class XII. In some of the prestigious colleges in our country due to the rush of applicants for B. Com. only the ones with high score in their class XII will only be getting admission. So, your marks in class XII are also important from that point of view.

Now coming to the career lines after XII, there are many options (including B. Com.) as follows -

(1) B. Com. - Commerce is very interesting and important academic line in our education system. This qualification prepares a person to work in the business and commercial environment. Commerce graduates can apply in the industrial arena in various capacities. The knowledge of commerce is required to work in the industrial set ups or can help an individual in starting his own commercial activity.

(2) B.A./ B.Sc. in Economics - This is one of the most lucrative stream after class XII and gives a career opportunity in investment firms, public sector undertakings, banking sector, education area, government enterprises and NGOs.

(3) BBA - It is a 3 years degree course in business administration and after this one can go for MBA or do some short courses in management area and then apply in the industry for suitable openings.

(4) B.Sc Computer Science - If you have interest in computer/ IT line then this is a very good option for career making in todays environment. It prepares the student with mathematical and theoretical foundations of computing. After this degree one can apply in IT companies or IT institutes for various jobs like software testers, developers, system managers etc.

There are numerous career options after passing out your class twelve examination and since you had been a student of science, you had abundant exposure in the science - stream.
However, you are no more interested in the present stream and would like to have a switch over the same wth the commerce.
However, to achieve success in any field, one must have a passion for it and would not mind the hurdles coming in the ways while persuing the same.
Now let us have the advantages of each of the courses-
B.Com - This degree is most suited to the aspirants having aptitude in understanding the different modules related to commerce such as Accountancy, Mercantile - law, Book - keeping etc. Commanding in such areas will enhance their knowledge in the area of commerce and later one can persue M.Com or can take up the career of CA or Cost - accountant with their dedication. Undoubtedly wide growth is possible in this area.
B.A - Studying the humanities with English, Ecoomics will surely provide you interesting plateforms and persuasion of either of the two would open up the chances of your growth in an incredible way.
MBA - A post graduation degree in Business - management with a renowned institute such as IMM or Xavier - Institute can widen your earning potential. This would be an ideal qualification once you pass out your graduation irrespective of the stream.

You have mentioned that you are science student and willing to switch to BCom, I would like to ask one question that Why are you planning to switch stream or what makes you to switch from Science to Commerce stream?
I always ask this question when students while choosing stream after X or XII. There are more open opportunities in Science stream compared to Arts or Commerce but it doesn't mean that Science is best stream. It always depends on your interest and it is really difficult to choose at younger stage at X/XII unless guidance is available.However as you have chosen to switch your stream I hope you have future plans or career thinking too.
As you have asked about Bcom admission you can have list of colleges and also eligible criteria for such colleges. You need to have your XII results for your admission which is preliminary criteria. Coming to point of interview questions, few colleges only can have marks criteria for admission but few colleges may ask question for choosing particular college compared to other colleges. In your case they may ask for switching stream from science to commerce. They may also ask for your future plans if any like career path if chosen any.
I am also giving you information about selective options or career in commerce. Commerce stream moreover focuses on either Accountancy or Competitive Exams like MPSC, UPSC or Other Government Sector exams which includes Banking, Railway, etc. As per above responses they have also shared about MBA and other options too but you need to chose among them upon your interest and what you want to be.
As you have planned to switch stream make sure you have better plans for career and upon that choose expertise or subjective subject for your graduation.