I am using MacVim on Mac OS X.
I want to know how to automatically input
<leader>u when I enter vim first time.
You can use
:normal (without the usual
!, so that the mappings apply), triggered via the
:autocmd VimEnter * execute "normal \\tt and \<C-e> and \\u"
To use the
<C-e> notation, you have to use double quotes and
\<Leader> doesn't work, you have to include the actual character (the default backslash must be escaped due to the double quotes).
If those are plugin mappings, there often are alternative custom commands that do the same; I would prefer those then, as it's more obvious than using the mappings.