I am integrating ripple for my trading website and what I know is that the ripple address is common to all the users but we need to provide destination tags to identify the user. So my question is how can I assign destination tags to the users and how can I know about the received transaction is of the corresponding user.Please Help.... :(
You can assign destination tags to users however you wish. You can even do it randomly, which is the recommended method. Just keep track of which destination tags you've assigned to which users so you know who to credit when you receive a payment.
For UTXO-based systems, you give every user their own address and you credit the user whose address received a payment. For XRP, you give each user their own destination tag and you credit the destination tag that received the payment. This method is much simpler because you only have to watch one address on the network.
Note that you should set the "destination tag required" bit in your receiving address to ensure that you don't receive erroneous payments with no destination tag. If you need to send funds to your own account for some reason, just use any unassigned destination tag -- zero is commonly used for this purpose.