Lately it is has been getting a lot harder. The problem right now is that so many things have moved so far, that there simply isn't a known pattern that will work.
Generally speaking, you now need a compiler. If not to install something other than a quite old system ruby, then to install common gems like nokogiri.
XCode seems to change significantly every release. Not long ago, you had to download it, or get it from your system CDs. Then you had to buy it from the app-store. Then you got it for free from the app-store. Then you got the installer for free from the app-store, and had to run the installer. And today I think you get the installer, which you run, and then you go through a series of menus to get to "install command line tools" (like gcc).
Once gcc is there, then you need a ruby. There's RVM and rbenv. Those need to be found, installed (by running a curl command piped through bash), and then a ruby needs to be installed. I use RVM, which is easy enough for me, and it does install bundler by default (which is good).
Once you have a modern ruby, you probably want a database other than sqlite. For that, you usually need homebrew. Again, install homebrew, and then have homebrew install postgresql/mysql.
At this point you are almost ready to install the rails gem.
Oddly, a lot of the newer complexity has less to do with Rails, and has more to do with the Ruby ecosystem getting both older and more complex at the same time.
Every time I help a new developer get started with the rails environment, I too am amazed at how much has changed since I setup my own laptop one year ago.
Generally speaking, you now need a compiler. If not to install something other than a quite old system ruby, then to install common gems like nokogiri.
XCode seems to change significantly every release. Not long ago, you had to download it, or get it from your system CDs. Then you had to buy it from the app-store. Then you got it for free from the app-store. Then you got the installer for free from the app-store, and had to run the installer. And today I think you get the installer, which you run, and then you go through a series of menus to get to "install command line tools" (like gcc).
Once gcc is there, then you need a ruby. There's RVM and rbenv. Those need to be found, installed (by running a curl command piped through bash), and then a ruby needs to be installed. I use RVM, which is easy enough for me, and it does install bundler by default (which is good).
Once you have a modern ruby, you probably want a database other than sqlite. For that, you usually need homebrew. Again, install homebrew, and then have homebrew install postgresql/mysql.
At this point you are almost ready to install the rails gem.
Oddly, a lot of the newer complexity has less to do with Rails, and has more to do with the Ruby ecosystem getting both older and more complex at the same time.
Every time I help a new developer get started with the rails environment, I too am amazed at how much has changed since I setup my own laptop one year ago.