Good, your phone has Bluetooth and supports MP3s. If you're unsure how to turn on the Bluetooth on your phone you'll have to consult its manual., but you should make sure the Bluetooth is on and the phone is discoverable.
To turn on Bluetooth on your MacBook: Go to System Preferences, then select "Bluetooth", then click 'turn Bluetooth on'.
To send the file to your phone, go to your Applications Folder, then Utilities, and then open up Bluetooth File Exchange. It should ask you for a file to send; select the MP3 file you want and click send. It will then ask you to select a device, click search and it should find your phone. Select your phone and click send. You will probably have to accept the file transfer on your phone.
Once the file is on your phone, you should be able to select it as a ringtone in your phone's settings.
It's also worth noting that if you select "Show Bluetooth status in the menu bar" in the Bluetooth preferences on your Mac it will give you a Bluetooth Menu which makes doing this regularly a bit quicker.