How to make it Secure?

I am building a website ( a platform ) which will have people communicate between each other and there is bound to be money transactions. The amount of money in each customers account will be kept in Database , This can be redeemed by the customer at any time ( ie: the website is liable to pay back whatever is the balance amount shown in the database ) Required Details: Database used : Mongodb How do i keep my database and the transactions secure ? Is there a problem with storing the account balance in database? Are there better and secure methods to store money in database Are there any concerns I should be worried about ? Are the any steps precautions to be implemented?