[gbm]

Originally Posted by Teeny

I have a folder in which I have all my books in there. Some of them are inside other folders, so what should I choose so that the books from all the sub-folders are added especially since some might have the same title but not the same author?

Also some of them are available in different format what can I do so only the MOBI is added?

You are new to calibre so I suggest that you stop and read the manual.

Quit trying to do your add books the hard way and do it the easy way. Right click add books and select option 2:

Add books from directories, including sub-directories (One book per directory, assumes every ebook file is the same book in a different format): Allows you to choose a directory. The directory and all its sub-directories are scanned recursively, and any ebooks found are added to the library. calibre assumes that each directory contains a single book. All ebook files in a directory are assumed to be the same book in different formats. This action is the inverse of the Save to disk action, i.e. you can Save to disk, delete the books and re-add them with no lost information except for the date (this assumes you have not changed any of the setting for the Save to disk action).

or 3:

Add books from directories, including sub-directories (Multiple books per directory, assumes every ebook file is a different book): Allows you to choose a directory. The directory and all its sub-directories are scanned recursively and any ebooks found are added to the library. calibre assumes that each directory contains many books. All ebook files with the same name in a directory are assumed to be the same book in different formats. Ebooks with different names are added as different books.

Just add all your ebooks to calibre then let it sort them for you.
example search:

format:mobi

will find all the ebooks that are in mobi format.

bernie