In Facebook, if I share some post from others timeline onto my timeline, who will be able to see it and where? Also who will be able to see it and where when I tag friends while sharing the post onto my timeline?
You can always set the privacy.
Once you tend to share a post it opens a popup in that we can have options to add extra content(Optional) about the post and also we will have privacy option to choose between
You can tag your friends by clicking on image and by choosing Tag option or simply you can tag your friends in the popup content by calling them in this way
From Facebook help center
If you want to change the audience of a post after you've shared it, click the audience selector and select a new audience. Remember, when you post to another person's Timeline, that person controls what audience can view the post. Additionally, anyone who gets tagged in a post may see it, along with their friends.