Hi, @nicolas-besnard glad to hear you want to switch to Vim, it s an awesome editor!!!
You are looking for YouCompleteMe - (Autocomplete Plugin). http://vimawesome.com/plugin/youcompleteme
By the way vimawesome.com is a great source for finding plugins for vim.
The guys are doing a great job, of providing you with an overview of available plugins for vim.
Did you check the Thoughtbot Laptop script, which setup your machine with a ruby/vim development environment?
When you are on a Mac is hassle free install. I use it on fedora 24 and also running into no issues while installing. It installs vim-ruby, vim-rails etc.
I hope I could help you! Feel free to write me, if you need help.
I am also a beginner, but I had a lot of “ohhhhh - awesome” and never looked back, after switching.
have a nice day!!!