The humble DJ mixer is the most important part of your DJ set up. Its sits between all your DJ instruments, controller, media players, CDJs or turntables. The mixer allows you to adjust the gain, volume, EQ, FX and fade between devices. Its also the only thing that will be use on every single mix you do!
There are several different things to consider when looking at DJ mixers.
Do you need a scratch mixer or a house mixer? - Are you going to learn to scratch? If so a scratch mixer with a very short cut-in time on the crossfader is the way to go here. With a larger fader curve its almost impossible to land tricks such as crabs, Twiddles and more intricate techniques. A dedicated scratch mixer will allow you to learn at a faster rate than trying on a house mixer. Another consideration is hampster (reverse) crossfader found on all scratch mixers and can be less common on house mixers.
Do you want rotary or regular faders? - An old but lately reintroduced fader is the rotary fader. Basically a large attenuation knob on each channel in lieu of a traditional fader. You will not find these on scratch mixers for obvious reasons.
FX or no FX? - You can save a bunch of cash if you don't feel like you'll need filters, delays, reverbs etc. But where is the fun in that?
How many Channels do I need? - Count how many CDJ's, Turntables etc you want to use simultaniously, thats how many channels you need. Factor in mic channels too if you will be announcing. Some mixers will have more mic channels than others.
Do you need built in DVS? - DVS stands for Digital Vinyl System. Some mixer makes such as Rane & Native Instruments for example have Serato or Traktor interfaces built into them. Purchasing this type of mixer can be a cost effective way to get both a DVS and Mixer in one rather than buy the two seperately.