I am tamil nadu.now I completed 12th. which would be better, BDS or computer science and engineering when I am completing my studies...?

There is no profession that you can outrightly reject as unsuitable. In you case, whether you go for BDS or computer science would depend on a number of factors:

• What is your passion in life. If you like serving people, BDS may be a good option. But if monetary benefits are your prime motive, BDS isn't the right pick. As for computer science, if you have a knack for science and like codes and all, it may be your thing.
• Your academic qualifications. If you are good in biology, along with Physics and Chemistry, it makes sense to go for BDS. But if instead you are weak in these subjects, especially Biology, and want to pursue the program just because of peer pressure or because your parents want you to, it isn't the right approach. Instead carefully evaluate your strengths and then take a decision. The same goes for Computer Science Engineering.
• BDS and computer science are very different disciplines. So, you must take into account the pros and cones of choosing one over the other.

Regards

Choosing a course totally depends on you and your interest. You should always choose the course according to your passion.

If we compare the scope of both the courses then CSE has better scope in the future in terms of jobs, placement and package.

CSE is the most demanding branch among students for engineering.

Job opportunities are available for Graduates in Government as well as Private sector. Graduates may take up the following job posts-

Software Engineer
Software Tester
Project Manager
Application Developer

Best colleges for CSE for placements are

IIT's
NIT's
BITS

Top companies that hire computer engineers:

Facebook.
Google.
Yahoo.
Microsoft.
Hewlett-Packard.
International Business Machines Corporation,
Toshiba Corporation.
Dell Inc.
HP.
NEC Corporation.
Canon Inc.
Apple Inc.
ASUSTEK Computer Inc.
Acer Inc.
Lenovo Group Limited
SAIC
Sun Microsystem
TCS
Infosys
HCL
Wipro
L&T
Tech Mahindra

Good Luck!!