i gave cat 2015 but did not get a good score. i want to give cat16. how should i start now?

Since you took CAT 15, you would know that CAT doesn't ask very difficult questions. So start with the basics and clear your concepts. Once you are done with this, you can start taking mocks. Mocks will help you understand your strengths and weaknesses so that you can work on your weaknesses and be better prepared when the actual exam comes.
We have a quite a few free mocks, you can try them. If you want more, you can also buy our test series to have access to further mocks.
Thanks.