There are simpler installers (and most probably guides) that will give you wordpress functionality on os x but if you want to get your hands dirty and do it yourself then read on.
Note: Whenever there is a line that starts with a $ you have to type the command into your Terminal (Found in your Applications folder) omitting the $.
A standard OS X install contains versions of Apache and PHP but not MySQL.
Apache (your webserver) needs to understand how to handle php code /private/etc/apache2/httpd.conf you can do this using the terminal like so:
$ sudo nano /private/etc/apache2/httpd.conf
Find the line: (ctrl+wphp5_moduleenter)
#LoadModule php5_module libexec/apache2/libphp5.so
Remove the # symbol to uncomment it.
Press ctrl + o to save.
Apache needs to then be restarted to reflect these changes, open system preferences go to sharing and untick and retick web sharing.
To install visit http://dev.mysql.com/downloads/mysql/ and download either the 32 or 64bit dmg and follow the instructions choosing the defaults (leave the root password as blank).
After it's installed you will have to create a database for wordpress to use.
Log in to the mysql console:
$ mysql -u root
Create the database:
> create database wordpress;
> exit
Download wordpress from http://wordpress.org/download/ and unzip into Sites/wordpress