Hi Moulika,
usually there are a lot of documents online with Q&A for BW interviews. Nevertheless, you need to know the core concepts of data warehousing, the steps to be followed,ETL, data cleansing and so on. After getting familiar with them, then you can explain all the above mentioned processes with the SAP BW info objects, info providers, transformations etc.
Hope this helps,
Thanks,
Regys