You can delete trailing spaces with the following command.

:%s/\s\+$//e

This command is explained as follows:

• enter Command mode with :
• do this to the entire file with % (default would be for the current line)
• substitute action s
• / start of the search pattern
• \s whitespace character
• \+ escaped + sign, one or more spaces should be matched
• before the line end $
• / end of the search pattern, beginning of replacement pattern
• / end of the replacement pattern. Basically, replace with nothing.
• e suppress error messages if no match found

You can delete all blank lines in a file with the following command: :g/^$/d

This command is explained as follows:

• enter Command mode with :
• g is a global command that should occur on the entire file
• / start of the search pattern
• the search pattern of blank line is ^g
• /end of the search pattern
• Ex command d deletes a line

You can convert tabs to spaces by doing the following:

First check that expandtab is switched off

:set noexpandtab

Then

:retab!

which replaces spaces of a certain length with tabs

If you enable expandtab again :set expandtab then and run the :retab! command then all the tabs becomes spaces.

If you want to do this for selected text then first select the text in visual mode.