Let's go ahead and get that fixed. Let me just say that you were smart enough to find the Apple community and post your issue here, so you're already a step further than someone who hasn't reached out.
You want to first go into System Settings. The icon is grey with a couple of gears in the middle. If you can't find it in your dock, click on Launchpad which is typically located on the far left of your dock and consists of 9 multi colored squares. When Launchpad opens up, there's a search bar at the top of the screen that you can type "System Settings" and it will show up under the search bar.
In the upper left corner of System Settings should be your name and "Apple ID". Click on your name and that will take you to the iCloud settings.
On the right side in the middle of the window, you will see an area that says "Apps using iCloud". The bottom of that section has a box that says "Show more apps...". Click on that
The first app on the list is Messages and if it says "on" click on that. The next window that pops up will have a green switch. If you click on the green switch it will turn off iCloud sync on your Macbook Air.
Be mindful of your storage on the MacBook Air, because photos and video files can add up quick. Click the back button located in the lower right corner and then the blue "Done" button in the lower right corner of the next window.
On the left, look for the section called "General". Click it once
On the right side, click on "Storage" which is the third item down
You can now see how much storage you're using on your MacBook AIr along with which specific apps are taking up the most space. You can also click on the Messages app in the list and see all of the file attachments saved on your MacBook Air along with deleting any large attachments you no longer want.