What can i do with an old mac book air?

Which MacBook Air do you have? To determine this, go to the Apple menu in the top left corner and select About this Mac. A window should appear and the model name of your MacBook Air should be displayed. For example, MacBook Air Mid 2013.

Knowing the model will determine what operating system the unit can take. The level of the operating system generally determines the viability of the device. Please note, MacBook Airs are generally designed NOT to be user upgradable. The unit's configuration is what it was at time of manufacture and this cannot be modified. Thus, you cannot add RAM or expand the hard drive. The exceptions to this are the 2010 through 2017 models. It is possible to replace the hard drive in this units. However, it may not be worth the cost as all of these models will soon be out of current OS support. (The 2017 model will be capped at Ventura giving it two more years of supported OS security updates).

Apple's latest operating system is macOS 13 Ventura. We are under two months away from the release of macOS 14 Sonoma. Apple officially supports the current minus two operating systems. So, today, Ventura, Monterey (macOS 12) and Big Sur (macOS 11) are supported operating systems. When Sonoma is released, Big Sur will be dropped from support. Ideally, you will want an Air that can support one of the supported operating systems. Yes, you can use the device on older operating systems, but understand those units will get no security updates and exploitable flaws may exist. In addition, you may experience compatibility issues with Apple's services and 3rd party applications (for example, Microsoft following Apple's current minus two strategy for the Office suite).

If your Air is a 2015 or higher, you still have some time. Please note, the 2015 model is capped at Monterey. Monterey will fall from official support in the Fall of 2024. This gives you another year of updates from Apple.

Anything older than the 2015, and you should be considering a replacement. The reason for this is the 2014 is capped at Big Sur. And as noted above, Big Sur will drop from support his Fall when Apple releases Sonoma.

Hope this is helpful,

Before we can address that , you need to tell us if anything is wrong with it.

Things like the exact year of the Mac and it's OS, problems, error messages, is it telling you to upgrade?

We can't just start offering solutions unless we know the condition of your Mac. If all this is beyond you, I suggest you ask an older person, parent maybe, to help you formulate your query.

This is a lot too read, but it tells you what you need to post to get good solutions: Writing an effective Apple Support Commun… - Apple Community