Azure offers free credit to students and accepts registration without a credit card. For this scenario, refer the following link for more information.
https://azure.microsoft.com/en-us/offers/ms-azr-0170p/
For other sign-up scenarios, including trial, you must provide a credit card. See the trial registration link below.
https://azure.microsoft.com/en-us/offers/ms-azr-0044p/
Once you sign up, you can use Azure budgets and cost alerts to ensure you get notifications before your free credit is completely consumed.
Hope this helps