Sound is as critical, if not more critical to the success of your video than image quality (see the attached example of sound with and without a microphone for an example of this). If your audience can't hear what you're saying, or if it the audio is marred by background noise, they will often lose patience and leave the video before you've had a chance to get your message across.

Luckily, this is a cheap problem to solve. There are two main types of microphones that we'll talk about for the purposes of a budget shoot. The first is the lavalier, or "lav" mic. This is the small mic that you see people on TV have tucked into their clothing, or attached to a lapel. Fancy versions are wireless, but you can get a decent wired lav microphone for use with a DSLR for around ~$20 at Amazon.com. Because the subject is wired up, this type of mic is best used for a sit down interview.

The second type of microphone is a directional microphone called a shotgun mic. This mic needs to be as close to the user as possible, without being in the shot, to get good audio quality. If you're walking after the subject, this may be a better choice than the lav mic. You can rent a Rode Shotgun Video Mic for 3 days for $12 at Borrowlenses, or buy it here at Amazon.

Either mic will likely work for your purposes, and it might make sense to get both if you have different setups (seated interviews and tracking shots) in your shoot.