If the screen was broken due to an accident or abuse, Apple will not replace it for free. The iPad's warranty covers manufacturing defects only. You can find Apple's charge to replace your iPad (Apple replaces iPads; they don't repair yours) and the procedure for getting it services by selecting your country here:
There may be independent repair services that would perform the repair at a lower cost. Search the web for "iPad repair" and you should find a number of such services.