First off, if you're already mixing tracks in your computer and you just need to combine your computer's output with a microphone, pretty much any mixer, DJ oriented or otherwise, will do.
But if you're planning to mix your tracks as well as the mic using the mixer, read on...
For less technically demanding DJing (such as at home or at a wedding) most any mixer targeted at DJs is probably fine - the most important components to look for are:
- A crossfader. Usually this is a horizontal slider, and it controls the relative level of two inputs. You use this to fade in one track while the other one is playing.
- A headphone jack. You use this to cue up other tracks, so make sure it can output a different mix than the master output. This way you can hear what you're about to mix in before the crowd does.
- A microphone input with a volume knob.
Almost every DJ mixer on the market has these features. Look for brands like Pioneer, Numark, Stanton, and Technics to start with, although I'm sure there are plenty of others.
There are other features you might also want to look at, but aren't as critical:
- An equalizer section on each input so you can adjust the tonal balance of a track. This is more important for club DJs but has its uses elsewhere.
- Volume level meters.
- Effects (either internal or external via an effects send).
- More inputs and routing options
You do get what you pay for, however - more expensive mixers will have less internal noise and more durable components, so the knobs and sliders will last longer. Wedding DJing is less demanding than, say, scratching and turntablism - and so this may be less important depending on what you're trying to do, but it's a consideration
If possible, try one out that you're looking at. Listen for how much white noise it adds to the signal, and try mixing through it - see if the control layout makes sense to you.