How do I fix a slow MacBook Air?

Install Etrecheck and post a report here and someone may be able to offer some solutions.

click here ➜ Using EtreCheck - Apple Community

How about some basics for us?

— Does it start?

— If it starts, do yo see any advisories or warning?

— Do you get a working desktop on startup?

— What function is "slow?' Starting? Opening apps? Something else?

Buying used Apple gear is fraught with peril because Apple's security measures inteneded to thwart theft can lock you out if the device was not properly prepared for sale.

Please understand that we are not Apple but your fellow end users. We cannot see nor remotely access your computer to find out which Macbook Air you have, its macOS version, or what kind of errors are occurring. In this setting we must rely 100% on what you can tell us.