You might wanna follow this tutorial here and redo it step by step that will give you a solid foundation with sequencer. Sequencer Basics | Unreal Engine Documentation
As for changing the camera focus distance that would be under point 2 - Your First Shot - Cameras
under point 9.
You can use the eyedropper to change the focus distance (so it will focus on the object you select with the eyedropper) or you can type the focus distance in manually.
After you’ve finished the quickstart guide above you might wanna look at those here, I use the crane nearly in every shot:
https://docs.unrealengine.com/en-US/Engine/Sequencer/HowTo/CameraRigCrane/index.html
and after you’ve get accustomed to those/if you want to look something up