Before we can setup PostGIS we need to have a local version of Postgresql installed and running. The most effecient way to do this is to download and install Postgres.app.
Once Postgres.app is installed in your computers /Applications directory. Open the Terminal and enter the following two commands
psql -d DATABASE_NAME -f /Applications/Postgres.app/Contents/Versions/9.3/share/postgresql/contrib/postgis-2.1/postgis.sql