if i download only free aps will my card be charged at any time

Only if you purchase something e.g. some apps allow you to make in-app purchases within them to unlock extra features (you can turn off in-app purchases on your phone via Settings > General > Restrictions > In-App Purchases 'off') or if you take our an auto-renewing subscription within it (e.g. newspaper and magazine apps). When you add or change the credit card details on your iTunes account then you might receive a small temporary store holding charge to check that the card details are valid and that the card is active, but that should disappear within a few days or so (store holding charge).