Gst for software export firm

We are a software services export firm. Since export of goods falls under zero rated supply , we are exempt from GST.

1) If we have to claim input tax credit , how do we do it ? Do we need to provide letter of undertaking or bond ? When is this letter of undertaking or bond needed ? Is it needed only when we have to take input tax credit or should it be done for general filing as well ?

2) We are paying tax for internet and rent for office. Can these be claimed as input tax credit ?