How can I add more storage to my iMac

Your tag line says that you have a 24" iMac running macOS 15.3 (Sequoia). That indicates that you have a 24" M1, M3, or M4 iMac.

You can't upgrade internal storage on any of those iMacs. So you add more storage using external drives.

Your iMac has several USB-C ports. These include multi-purpose "Thunderbolt / USB4" ports, marked with a small lightning bolt symbol, that can handle USB 3, USB4, DisplayPort, and Thunderbolt. They will adapt to whatever you plug in. They may also include single-purpose USB 3 ports that are not marked with a lightning bolt symbol. (If you need to go between USB-C and USB-A to attach a USB 3 accessory, there are many ways to do that.)

So you have a lot of choice in drives, including

• Solid-state drives (SSDs) of varying speeds and capacities
• Bus-powered hard drives, with capacities of up to about 5 TB
• Wall-powered hard drives, with capacities of up to about 20 TB

If you are looking to free up space, you can move your Photos, Music or Movie Libraries to an external drive.

Move your Photos library to save space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

Move your iMovie for Mac library - Apple Support

If you go this route, I recommend using high quality external drives instead of cheap off the shelf external drives.

https://eshop.macsales.com/shop/external-storage#drive-configuration:single-bay-1-drive

I also recommend that you use at least 2 external drives to safely protect your media Libraries.

one for your media Libraries and a second to backup the media Libraries.

If you are a typical user what takes up the largest amount of data on your machine are the following libraries:

• Photos
• Music
• Movies

Locate these libraries on the computer and move them to the external drive you decide on. Apple explains this process for the Photos library, the process is the same for Music and Movies. Please study Move your Photos library to save space on your Mac

Obtain an external Thunderbolt SSD. OWC / MacSales is a good source.

If speed is not important but budget is consider this type of setup:

Just some food for thought.