In command mode(Esc) enter :gg=G to use Vim's built-in indention engine.

Command Part	Description
gg	start of file
=	indent (when equalprg is empty)
G	end of file

You can set equalprg in your .vimrc to use a more sophisticated auto-formatting tool.

For example, to use clang-format for C/C++ put the following line in your .vimrc file:

autocmd FileType c,cpp setlocal equalprg=clang-format

For other file types, replace c,cpp with the filetype you want to format and clang-format with your preferred formatting tool for that filetype.

For example:

" Use xmllint for indenting XML files. Commented out.
"autocmd FileType xml setlocal equalprg=xmllint\ --format\ --recover\ -\ 2>/dev/null
" Tidy gives more formatting options than xmllint
autocmd FileType xml setlocal equalprg=tidy\ --indent-spaces\ 4\ --indent-attributes\ yes\ --sort-attributes\ alpha\ --drop-empty-paras\ no\ --vertical-space\ yes\ --wrap\ 80\ -i\ -xml\ 2>/dev/null

To indent our outdent the current line in normal mode press the greater than > key or the less than < twice accordingly. To do the same on multiple lines just add a number beforehand 6>>

Command	Description
>>	indent current line
<<	outdent current line
6>>	indent next 6 lines

You can also indent using motions. Here are a few useful examples.

Command	Description
>gg	indent from current line to first line in file
>G	indent from current line to last line in file
>{	indent previous paragraph
>}	indent next paragraph

In visual mode by pressing the greater than or less than key just once. Note that this causes an exit from visual mode. Then you can use . to repeat the edit if you need to and u to undo.