Are you an ex-MobileMe subscriber who has the complimentary extra storage? If so this will downgrade to the free storage level at the end of next September, and if you want to keep the storage you will need to cancel the downgrade and pay for the additional storage. Note that if you purchase additional storage before next September it will cancel the complimentary storage immediately. Please see
If you are not an ex-MobileMe subscriber then as said above you need do nothing except make sure you have a valid credit card registered.