Combined Defence Services examination paper has three sections- English, General Studies and Elementary Mathematics. We have last few years' question papers of CDS which you can download from the below link. Try to solve these papers by your own so that you can understand how much prepared you are. Then check your answer with the answer keys. Solving previous year exam papers' help you to understand the exam pattern and what type of questions are actually asked. You can download the papers from here
http://competition.careers360.com/articles/cds-question-papers
You can download the answer keys of respective years from the official wesbite http://www.upsc.gov.in by selecting the year.
CDS Syllabus is as follows-
Algebra, Trigonometry, Mensuration, Mean, Median and mode, Time and distance, Number system, Simple and Compound Interest, Profit and loss, Allegations, Percentage, Time and Work, Set theory, Clock, Geometry.
General English Grammar and Vocabulary, Unseen passage, Cloze test.
History, Geography, Science, Ecology, Politics, Economy, Armed Forces.