Form 12BB is the statement of claims by you, as an employee, for deduction of tax. This will be collected by your employer and has little to do with Income Tax filing directly. Note that with effect from 1st June 2016, you have to submit the Form 12BB to your employer (if you are salaried) to claim tax benefits. This is also applicable to the rebates that you claim on investments and expenses.
Form 12BB is applicable to all salaried taxpayers. You will have to submit Form 12BB at the beginning of every financial year. If you join a new firm, you will need to provide this as soon as you join. Using this form, you will declare the investments you plan to make and the expenses that are eligible for tax deductions that you might incur. You should provide documentary evidence of all the investments and expenses at the end of the financial year. However, most firms have their own version of Form 12BB that they use it to collect this information from employees. So, speak to your employer about this.