The simplest way is to use either Quicktime (simply choose File>New Audio Recording) or a free and open-source app like Audacity. Your MBA has a built in microphone, so all you need to do is click record and then start your conversation.
If you do this regularly, it way be worth investing in a USB mic that has better sound quality than the built-in mic on your Macbook. Something like the Samson Go Mic is fairly well regarded for field recordings, dictation, and interview recordings.
Lastly, transcription can be a huge pain without software that supports easy pause/resume/rewind capabilities. There are a couple free ones out there, like Transcriptions or more expensive commercial solutions. (Hint: Get a foot pedal!)
Sound-to-text technology is getting increasingly more accurate (though not foolproof as anyone who uses Google voicemail transcription knows), so depending on the quality of your source material and the time you spend training, you can also eliminate a large chunk of the manual transcription with something like MacSpeech Scribe