Well, it is not that simple.
What you can come up with -- use syntax definitions. But this bold text you will create would be temporary, it will not be saved with your file like word processing software does.
So to the code.
hi MyBold gui=bold
let l_start = getpos("'<")
let l_end = getpos("'>")
let c_start = getpos("'<")
let c_end = getpos("'>")+1
execute 'syntax region MyBold start=/\%'.l_start.'l\%'.c_start.'c/ end=/\%'.l_end.'l\%'.c_end.'c/'
vnoremap <leader>b :<C-U>call RegionMakeBold()<CR>
Having this in your
.vimrc will allow you to make bold text out of visually selected blocks. Just select text, press
<leader>b and text should be bolded.
It will probably not work for the files with defined filetypes/syntax.