I am locked out of my MacBook Air, it’s pretty new to me so I am not sure what to do?

Hello , i have Apple Watch and phone , but now I have MacBook Air and it’s all new to me . This is the second time I have been locked out of my MacBook Air. Can someone tell me what to do? I have a recovery key from last time, but I don’t even know how to get into the Mac to put it in if it’s still good .Thank you.