The file .vimrc in your home directory has instructions for [[vim]] to load every time it runs. Customizations go there.
For instance, you might want to use [[Markdown]] to edit files that end in .md. The system default for some reason is to treat these as [[Modula-2]] files, though we don’t have a Modula-2 compiler running (yet). So the contents of .vimrc should read
au BufRead,BufNewFile *.md set filetype=markdown
For more suggestions in deep depth on how to set up your .vimrc please read this tutorial from Doug Black.