Can't install rails on my new Mac OS X Lion 10.7.3

Just got a new macbook pro and im that has problems installing rails .. I already have ruby ​​1.8.7 installed, and when I try to write 'gem install rails', I get:

ERROR:  While executing gem ... (Gem::FilePermissionError)
You don't have write permissions into the /Library/Ruby/Gems/1.8 directory.

I also tried installing a higher version of ruby, and he said:

    Nirs-MacBook-Pro:~ nirohayon$ rvm install 1.9.3
Fetching yaml-0.1.4.tar.gz to /Users/nirohayon/.rvm/archives
Extracting yaml-0.1.4.tar.gz to /Users/nirohayon/.rvm/src
Configuring yaml in /Users/nirohayon/.rvm/src/yaml-0.1.4.
Error running ' ./configure --prefix="/Users/nirohayon/.rvm/usr"  ', please read /Users/nirohayon/.rvm/log/ruby-1.9.3-p125/yaml/configure.log
Compiling yaml in /Users/nirohayon/.rvm/src/yaml-0.1.4.
Error running 'make ', please read /Users/nirohayon/.rvm/log/ruby-1.9.3-p125/yaml/make.log


Database file /Users/nirohayon/.rvm/config/packages does not exist.

Installing Ruby from source to: /Users/nirohayon/.rvm/rubies/ruby-1.9.3-p125, this may take a while depending on your cpu(s)...

ruby-1.9.3-p125 - #fetching 
ruby-1.9.3-p125 - #extracted to /Users/nirohayon/.rvm/src/ruby-1.9.3-p125 (already extracted)
ruby-1.9.3-p125 - #configuring 
Error running ' ./configure --prefix=/Users/nirohayon/.rvm/rubies/ruby-1.9.3-p125 --enable-shared --disable-install-doc --with-libyaml --with-opt-dir=/Users/nirohayon/.rvm/usr ', please read /Users/nirohayon/.rvm/log/ruby-1.9.3-p125/configure.log
There has been an error while running configure. Halting the installation.
ls: /Users/nirohayon/.rvm/rubies/*/bin/ruby: No such file or directory

PLEASE help me, I used windows earlier, and also quite new for rails, so I don’t know much about which way I should direct my terminal. I need a step by step explanation to put the rails and stand on my mac.

Many thanks! Nir