Vim's defaults for :split and :vsplit are to open the splits above and
to the left, respectively. I prefer for them to the split below and to the
right, respectively. To get :split to split below:
:set splitbelow
and to get :vsplit to split to the right:
:set splitright
Add those two lines to your .vimrc to have them set that way all the time.
If you have them set as such in .vimrc and want to temporarily revert the
setting for the current file, type:
:set nosplitbelow
or
:set nosplitright
as commands.