The easiest way to make your guitar talk is to buy a device called a "Talk Box". There are different companies that make talk boxes. One of the best known is the Heil Talk Box. Bob Heil from Marissa Illinois makes many devices for audio and radio applications. He got his start making large public address systems for big rock concerts.
You may have heard an example of a talking guitar if you have heard songs like Peter Frampton's "Do You Feel Like I Do", or other songs where by all accounts it sounds like a guitar is actually talking.
The basic idea of a talk box is that the sound from your guitar is channeled through a plastic tube and into your mouth. You then use your mouth as a dynamic filter to form words from the sounds emanating from your mouth. Normally, your voice box produces sounds that are then shaped into words by the shape and movements of your mouth. With a talk box like the Jim Dunlop HT1 Heil Talk Box or the Rocktron Banshee 2 Talk Box you are substituting the sound of your guitar for the sound produced by your voice box.
Because the sound that is produced by a talk box is coming out your mouth, rather than through your amplifier, you need to amplify the sound that comes out of your mouth. This is done by using a microphone that is amplified by a public address system or by some other amplifier.
In practice, the way you use a talk box is to plug the cable from your guitar to the input of the talk box. The talk box amplifies the sound of your guitar and the output goes into a flexible plastic tube. The normal way to use the tube is to tape it to a microphone stand so that it runs up the microphone stand to the level where your mouth is. Then you stand by the microphone stand and place the end of the tube in your mouth. Since there is a microphone on the microphone stand, it serves as the way to amplify the sounds coming out of your mouth. The microphone is connected to the public address system.
When you play a note on your guitar you can change the sound of it by changing the shape of your mouth. Just like your voice box can produce sounds of different volume and pitch, you need to learn to play the correct volume and pitch on your guitar to make different words.
You may also choose to have the unaffected portion of your guitar amplified as well.
Try to practice listening to your own voice as you make the words you want to produce. Listen to the changes in volume and pitch as you make the words. Now try to emulate those same changes in volume and pitch on your guitar by playing notes of those pitches. You will usually be playing single notes at a time, but don't be afraid to experiment with playing chords or other configurations on your guitar as you use your talk box. There is no real right or wrong way to use your talk box. Experiment and practice and find how you like to use it. With practice you can also make your guitar "sing"!
It does take practice, but the first step in making your guitar talk is to purchase a talk box. Then practice, practice, practice!
You can simulate making your guitar talk by using other effects, such as a wah wah pedal, but to achieve the effect that was made famous by Peter Frampton and others, you will need to buy a talk box.