Do you use Reminders on your iPhone? A little knowledge is usually all it takes to use them effectively, particularly so far as Siri is concerned.
Sync them up
You can choose to keep your Reminders solely on the device – this can be useful if some of your Reminders are private, or if you are setting work-based prompts on your business iPhone, but most people will choose to sync them across all their devices. Sync is controlled here: Settings>iCloud>Reminders, set to on (green). When this is enabled your Reminder will appear across all the devices you have logged into your account.
How to add a reminder manually
Open up your Reminders app and select the appropriate Reminders list. Once in the list you can type (or dictate) you reminder and edit its details by tapping the small circle-surrounded ‘i’ icon that appears to the right of the item (see below).