fan on macbook air keeps coming on and staying on

Post a screenshot of Activity Monitor if you wish. Use the Energy pane, or CPU sorted by %CPU to see which apps or processes are using the most energy or placing the greatest demands on your Mac's CPU, respectively. To show the most relevant information I suggest selecting "Active Processes" from Activity Monitor's View menu.

It could be that there are background tasks running that get your Mac hot. You can check the background tasks that are running via the activity monitor