You could find the subscriptionId in the Azure portal or by using Azure PowerShell.
For the Azure PowerShell, you could use the cmd "Login-AzureRmAccount" and provide your Azure credentials (email address and password).
When you are logged in, all information of your default subscription will be shown and you can find your Subscription ID in the SubscriptionId field.
When you have more than one subscription you can find the Subscription ID of a specific Subscription by typing "Get-AzureRmSubscription". This command will list all of your subscriptions, with details like the SubscriptionId, which are associated with the logged in account.