Basically there are two ways.
- Use a synthesizer and create a sound that sounds like a vocal sound.
- Use a real voice and process it so that it sounds more "computerized"
1: Use a synthesizer and create a sound that sounds like a vocal sound.
When you use a synthesizer that basic way of creating a vocal like sound is to have a sawtooth oscillator and a low pass filter after it. Then you apply a LFO to the filter cutoff frequency. The LFO Frequency has to be pretty high and a lot of synths actually have not fast enough LFOs. Then the modulation depth (LFO to Filter), cutoff frequency position and the LFO speed actually shape your vocal sound. It really needs fine tuning to get it sound like a vocal. Sometimes I even can't recreate my own vocal sounds which I created this way.
An easier solution is to use so called "vocal filters" or also "vowel filters" or "formant filters" as they apear in synths like the nord modular series. They will make a vocal sound out of everything. There are also some plugins who actually can do that.
Also good for vocal like sounds is to apply strong phaser effects with a lot of stages and nicely tuned feedback but without using the LFO which all phasers have.
2: Use a real voice and process it so that it sounds more "computerized"
The other way is to actually sing, use a real voice and apply effects to this.
The obvious effect is the "cher" effect well known from the Cher hit "do you believe in love" this is done by using a pitch correction plugin like "Antares - Autotune" or "Melodyne". The trick is to abuse such a plugin and set it up in a way that it corrects the pitch totally unnatural, which leads to a computerized but funny voice sound.
Some of those pitchshifters actually accept midiinput, so that you can use a midikeyboard to actually define the melody to which the voice sound has to be pitchshifted.
Another possibility is to use vocoders. Basically you "wrap" a synth sound aroung a voice. Therefore of course the melody you hear will be the melody of the synth sound, but the "shape" of the sound will be your vocals. Vocoders are good fun to be able to wrap any sound around anysound.. for instance to wrap a pad around a drumloop in order to make melodic rhythms. A very compley vocoder is for instance: Native Instruments Vokator.
Example:
Here some vocal manipulation example. I used the voice processor in cubase to do some vocal stuff in this song:
http://www.subconsciousmind.ch/media/releases/intermezzo/SubConsciousMind-Intermezzo-128/02-(Intermezzo)-SubConsciousMind(SCM)-MyMusic.mp3
They start at 2.31 Minutes and evolve till 3.20.
I simply fed my vocals into the plugin and controlled the melody via a midikeyboard. Then I did some weird delay stuff with Waves Multitap.