What you need is called a talk box.  It's often referred to as a talkbox, vocoder, or voice synth.  The best brand out there is Banshee Rocktron Talk box.  It'll run you about $150, but it is totally worth it.  It is a lot like a stomp box or distortion pedal for a guitar, but you can use these for any instrument or just your voice.

They are easy to hook up.  All you need is the talkbox, the power cord, and your input/output(such as mic, amp, guitar, etc.)   What you do is plug it in, plug the amp in on one side, and your guitar or mic on the other side.  You're good to go!

Rocktron Banshee Talk Box (w/Modded Driver)    US $125.00

ROCKTRON BANSHEE 2 TALK BOX GUITAR EFFECTS PEDAL    US $179.99

Rocktron Banshee 2 Talk Box Guitar Effects Pedal New!    US $189.00

Rocktron Banshee Talk Box Guitar Effect Pedal    US $75.00

Rocktron Banshee 2 (Amplified Talkbox)    US $219.00

Rocktron Banshee Talk Box    US $169.00

ROCKTRON BANSHEE TALK BOX GUITAR FX EFFECTS PEDAL,BOX AND MANUAL,GREAT SHAPE    US $129.00

Rocktron Banshee Talk Box Guitar Effects Pedal - NEW!    US $144.50

Rocktron Utopia G200-B Multi-Effects Processor w/integrated Banshee Talkbox NEW    US $379.00

Rocktron 9v AC Hush, Xpression, All Acess, Blue Thunder, MIDI Mate, Banshee NEW    US $19.99

ROCKTRON MOUTHPIECE FOR BANSHEE    US $23.99

Rocktron Banshee Talkbox 11254 Guitar Pedal talk box wah    US $184.95

Rocktron Banshee (Amplified Talkbox)    US $159.99

Rocktron Banshee Talk Box Talking Guitar Effect Stompbox Talkbox BRAND NEW!    US $139.99

ROCKTRON EUROPEAN POWER ADAPTOR BANSHEE GAINIAC MIDI MATE 220 VOLT AC VOLTAGE    US $39.99

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