First, you need to understand the difference between MIDI, digital audio and an analogue electrical signal.
MIDI is a sequence of instructions like "start playing middle C at volume 200", "stop playing middle C", "shift the pitch +3" and so on. MIDI is generated by things like keyboards and sequencers, and understood by things like synthesisers and drum machines; what GarageBand calls "software instruments". It's probably not relevant to your guitar.
An analogue electrical signal is what comes out of a microphone, an electric guitar, or the transducer in your electro-acoustic guitar. It's simply a continuously varying voltage, going up and down in the same way as the sound waves that you eventually want to hear.
A digital audio signal is the computer's representation of the analogue signal - the analogue signal "sampled" into a sequence of numbers.
What you are asking, is how to convert the analogue signal from your guitar into the digital audio signal your computer needs.
Note that what you have is an electro-acoustic guitar. If it was just an acoustic guitar, it would not have any kind of socket.
First, make sure the transducer in your guitar works. A transducer is a lot like a microphone or pickup, except that it detects vibrations in the surface it is attached to, instead of having its own diaphragm or detecting vibrations in metal strings. Borrow an amplifier, plug your guitar into it, and verify that sound plays from the amp. Like microphones and pickups, some transducers are "active" and need a battery. Others are "passive" and don't need a battery. If there is no obvious battery compartment, it's likely yours is passive.
Next, you need a suitable socket in your computer. There are a few options here:
- If your computer has a microphone socket, you can use that. It may not be ideal, but it will work
- If your computer has a line-in socket, you can use that. However, the signal from a transducer/microphone/pickup is too quiet for a line-in. You need to first amplify the signal to line level. There are several options for this. Anything that can drive headphones is good enough -- a pocket practice amp, a multi-fx pedal etc.
- If your computer has no built in audio input, you can add one. You can fit internal sound cards to desktop systems, or you can get USB sound interfaces. The Griffin iMic is a compact and affordable USB option. You can pay more for fancier interfaces with more inputs, professional connections (such as balanced microphone sockets), better sound quality etc.
Before trying to use these inputs in a program such as GarageBand, check that they work in the OS's control panel. This varies depending on your OS, but in both Mac and Windows, there are sound control panels where you can select an input, and watch a signal bar respond to the sound you make.
Once you know it is working, and you know the name the OS is using for the input your guitar using, telling your music application to use it should be straightforward.
Hint: in GarageBand, choose either "electric guitar" or "real instrument". Even though your instrument is electro-acoustic, follow the GarageBand help pages for an electric guitar.