There are multiple case in Unity where you want to use sprite sheets.
The most common scenarios are:
Importing sprite sheet with Unity is simple, just drag and drop the sprite sheets in the resource folder.
If you need a spritesheet you can test with this one : sonic-sprite-sheet.png
Then make sure the Sprite mode is set to Multiple :
Then you can open the Sprite Editor :
You can slice the sprites, normally you do not need to tchange the default options :
If the sprite sheet has been sliced correctly you can click on Apply to confirm the cghange or Revert.
After the slicing is done you should have :
In the resource folder you see the sprites :
Everything is done you can use the sprites in your game!
If you want to reduce the number of draws calls but you have multiple sprites in your games, you can generate a sprite sheet with unity, those are call Sprite Atlas.
To create a Sprite Atlas you can do it in less than 1 minutes : Right click on you resource folder
Then you can add the sprite you want to pack in the Inspector:
You can render you Sprite Atlas with the button Pack Preview.
Unity should be able to use your Sprite Atlas you do not need to do anything else.