Im trying to update to MacOS Monterey and it says there is not enough space on the selected disk how do I add space or buy more storage so I can download

The internal drive size is fixed, and you cannot change that physical constraint. You can start by cleaning out big files in your home directory such as your downloads directory, looking for email with huge attachments, or relocating your Photos library to an external drive to name a few options. If you are upgrading from macOS 10.14 Mojave, then you will need 36 GB of available storage in addition to the size of the Monterey installer (12.4 GB).

Empty your Trash. From the Apple () menu > About This Mac, select the Storage tab. On that panel, click Manage… which will present you with yet another panel. Select the Review Files button under Reduce Clutter. This will show you files in descending size under categories as Large Files, Downloads, Unsupported Apps, Containers, and File Browser. I would focus on the Large Files, and Downloads categories.