How do I start vim in insert mode?

You can start vim like this:

vim -c 'startinsert' FILENAME

If you want, you can edit the .bashrc file (if you are using bash) and add this line:

alias vim="vim -c 'startinsert'"

You can use vim +star, which is even shorter. NB: star is short for :help :start.

If you want this behavior by default, the best option is to add the line

autocmd BufRead,BufNewFile * start

into your ~/.vimrc. Also take a look at :h 'insertmode', which outlines a special option made for this kind of functionality. However, it can make it difficult to get out of insert mode which is crucial for growing in your vim ninja skills.