Open the Terminal program (this is in your Applications/Utilites folder by default). Run the following command
touch ~/.bash_profile; open ~/.bash_profile
This will open the file in the your default text editor.
For ANDROID SDK as example :
You need to add the path to your Android SDK platform-tools and tools directory. In my example I will use "/Development/android-sdk-macosx" as the directory the SDK is installed in. Add the following line:
export PATH=${PATH}:/Development/android-sdk-macosx/platform-tools:/Development/android-sdk-macosx/tools
Save the file and quit the text editor. Execute your .bash_profile to update your PATH.
source ~/.bash_profile
Now everytime you open the Terminal program you PATH will included the Android SDK.