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.