what connector do i need to use an external hard drive for iPad

It depends what interfaces/ports your devices have (both the iPad and the external drive)

I think it will mostly be this one: Lightning to USB Camera Adapter

But,

Depending on your model, use the Lightning to USB Camera Adapter, Lightning to USB 3 Camera Adapter, USB-C to SD Card Camera Reader, or Lightning to SD Card Camera Reader (all sold separately).

Note: The Lightning to USB 3 Camera Adapter can be powered with a USB power adapter. This allows you to connect USB devices with higher power requirements, such as external hard drives, to iPad.

To get all the details read this article: Connect external devices or servers with Files on iPad - Apple Support

For iPadOS13.x to access external media, it must be formatted as FAT, FAT32 or exFAT (FAT64) for universal readability across Apple, Microsoft or Linux systems - or additionally APFS if only used with Apple systems. NTFS and other formats are not supported. Drive formatting must be performed on other computing devices; drive formatting actions are not supported by iPadOS.

Additionally, for compatibility with iPadOS, the drive must only have only single partition.

HDDs tend to more problematic with iPadOS/iOS13.x than flash-based storage media - such as USB thumb-drives. You will also need to provide additional power to the HDD via a powered USB hub - as the iPad will not be able to provide sufficient power.

Dependent upon your model of iPad, you will need to use an appropriate adapter (such as the Apple Lightning to USB3 Camera Adaptor - for devices that use a Lightning connector) or directly using USB-C for the most recent models of iPad Pro. When using USB-C, you may still need to use a self-powered HDD or powered USB hub to provide adequate power to the system.

Connect external devices or servers with Files on iPad - Apple Support

I hope you find this helpful.