Contact a local band or two in your area, and ask if they'd like to have you shoot a music video for them. If word gets around, you'll soon have more work than you can handle.
Plan the shoot. Meet with the band ahead of time to discuss possible concepts for the video, to solicit their feedback and ideas, and to hammer out the shooting schedule.
Share your vision. Will this be a straight concert performance piece? Or do you envision the video as a psychedelic explosion of color and melting visuals, with the band dressed in '60s throwback clothing? The working relationship will go much smoother if everyone is on the same page and working towards the same unified vision.
Storyboard the video in advance to ensure no one is surprised or upset by your shot selections. (Some people are very sensitive to being photographed from certain angles.) Many free or low-cost storyboard programs are available for the Mac, and filmmakers should take advantage of them.
Rent, borrow or purchase your equipment. A video camera that shoots in 24p mode (24 frames per second, the same speed as a film camera) will go a long way towards disguising the low-budget origins of your project. Since your music video will feature pre-recorded audio supplied by the band or artist, you won't need to spend a dime to record sound. However...
Make sure you have plenty of light! Poor lighting is one of the biggest indicators of an amateur production. Small lighting kits are available for rent from most camera supply stores. If your shoot takes place outside in sunlight, have plenty of white posterboard or styrofoam bounce cards to reflect light onto the faces of the musicians.
Record your performance footage first. Play the song loudly on a CD player (out of shot), while the whole band mimes playing the song in its entirety. Repeat the process in close-up or medium shot for each member of the band, ensuring that you have lots of footage of your singer. This way, if anyone in the band decides to play diva and walk off the set after the fifth play-through, you have enough footage in the can to proceed without any further participation from this person.
Obtain any non-performance footage (i.e., story sequences, crowd reaction shots) last. With the repetitious "pretending to play the same song over and over" shots behind them, the band may become re-energized. Take advantage of this by saving your story sequences until the end.
Make sure you have enough cut-away footage before you leave your shooting location. Cut-aways help cover awkward edits and can add power to your presentation. Grab shots of the audience cheering for the band, band members horsing around together, interesting features of the shooting location, etc. If in doubt, shoot a little more.
Edit the video. Since you're using a Mac, you're in luck: you probably already have a copy of iMovie installed on your computer. This basic software is sufficient for most editing needs, allowing the ability to cut and rearrange shots, a number of different video transitions like dissolves and fades, and an easy-to-grasp drag-and-drop interface that can be learned in a matter of minutes. If you need more than simple editing functionality, Apple provides two excellent (albeit expensive) solutions: Final Cut Express ($299) and Final Cut Pro ($999). Final Cut Express provides a more advanced (and industry standard) timeline editing system, which allows immense control over both audio and video. Powerful color correction tools are part of this software, which can enhance your creativity---you literally have the power to make the grass impossibly green, or give every member of the band blue Smurf-like skin tones. For the ultimate in Mac editing power, Final Cut Pro is the king of the hill. With an interface almost identical to Final Cut Express, Final Cut Pro adds the ability to edit 1080p HD video, and the direct utilization of Adobe After Effects filters, which are used in many special effects-heavy feature films. If your video was shot on digital video (DV), then Final Cut Pro is probably more power than you need, but it's an option if you have a large budget and the ambition to match. NOTE: Whichever editing package you use, make sure to lock your band's audio track, so that your editing affects only the picture and not the song itself! Otherwise your final song will sound like it was placed inside a blender and randomly reassembled.
Solicit feedback from the band as you complete each cut of the video. It's your vision, but it's their music. Keep working on it until everyone agrees it's great.
Distribute the finished video. The purpose of a music video is to publicize a band or artist; as such, you'll want to seek the largest possible distribution for the final product. If the band is not signed to a record label, work with the group directly to establish a distribution strategy. Can the video be uploaded to YouTube? Will they host it on their website? Will they burn DVD copies to hand out or sell at live shows? Can you get the video into the Apple iTunes store? MTV plays increasingly fewer music videos these days, but you might get some airtime from local television or public access stations. Think creatively!