As you know, Azure Resource Manager (ARM) is the default deployment model for Azure resources but if you had already deployed Azure resources using the old model/portal (Classic deployment) it was not easy (not to say impossible) to easily migrate these Classic resources to the new ARM.
Now, you can 
If you have classic resources (like virtual machine, network or storage account) you can now use the Migrate to ARM option available in the configuration blade of these resources.
The migrate to ARM option will validate the capability (there could be still some incompatibility) and then (if validated) performs the migration
Off course the validation options differ depending of the resource type to migrate.
Happy ARM migration.