First, you have to pass your 12 standards by appearing in all subject. You can not give the examination of only one subject Bookkeeping and Accountancy. You can not appear as a regular student, you have to complete your study by appearing as a private candidate. I will suggest you to do your intermediate through NIOS. The complete procedure for the NIOS is below:
1. Register yourself on NIOS by visiting this site
. After registration, you will have your password and user ID.
2. Use this user ID and Password to fill the form.
3. After filling form you have to pay the fees of Rs. 1500. You can pay through Net banking or Debit / Credit card.
4. After the payment, print the application form and send to the regional office with the required documents. You need Adhaar card, 10 Marksheet, etc.
5. After submission of your application, NIOS will send you the books and you can also download this syllabus and books through the official site of NIOS (www.nios.ac.in).
6. You will be provided with a study centre contact this study centre. You have to submit your assignment to this study centre.
7. You have to complete your intermediate in four years.
After completing your intermediate, you can get B.Com degree through IGNOU. But first, concentrate on 12 and after B.Com.
I will provide you further detail if necessary. If you need any help give a response in this thread.