Will vastly improve your audio. I used this on my last two DVDs. The Audio-Technica ATR-35S Lavalier Microphone. $19 on Amazon.
You’ll also probably want to get a 3.5mm Stereo Male to 3.5mm Mono Female Adapter. It doesn’t produce stereo sound (you don’t WANT stereo sound for dialogue in a movie, even Hollywood records talking in mono), but it will record your mono mic as two tracks of mono, rather than one track, which will save you a lot of pasting and panning in the editing. Here’s one for five dollars.