Have a look here > http://www.macprovideo.com/tutorials/garageband-application
I'm thinking about getting into all this...
I think you should expand on this comment a bit more and give some details as to what you would like to achieve. That way we can see if GB will fit your needs and to what extent you would invest in gear or software. There are quite a few things to consider when starting out and you can easily blow a budget as well as regret a purchase or two...
"I want to hook up a digital keyboard (don't have one yet since I play on a Grand Piano), a guitar for when my buddy comes over, and mic."
I don't want anything expensive because my goal will be simple, I just want to record myself playing on a digital keyboard and also have the ability to have 2 additional hook ups for possibly an electric guitar and mic. I've been asking around and someone told me they use a Mbox 2 Pro but I don't know if that's too much for what I want to do.
Most people buy an external soundcard for two reasons 1. Better quality Audio to digital and Digital to Audio converters than the on board sound card of your computer, and 2. Connectivity options for various pieces of equipment that do not come standard with a computer.
In your case, you could get away with everything being USB by using a Keyboard such as an http://www.m-audio.com/products/en_us/Oxygen61.html or an http://www.m-audio.com/products/en_us/Axiom61New.html or even an http://www.m-audio.com/products/en_us/Oxygen88.html. These are simple to connect and have some built in GarageBand integration.
These pieces of gear are simple to use and would get you started easily.
As suggested above in previous posts, there are many brands of external sound cards you could also use if you wanted something more substantial. Probably the biggest things to consider are if you are going to use FireWire or USB to connect the sound card, and wether you are going to opt for a microphone that requires phantom power vs a mic that does not. For vocals, many people choose large diaphram condensor mic's that require extra power and for electric guitars best results are obtained with 'High - Z' connectors, inputs optimised to recieve a signal from a guitars electrics vs 'Line Input' which is the signal that would come from say and external synth.
The best advice I can give is to not buy twice... It really is worth doing some research and shelling out reasonable money first time round. Buy into a brand that has a good reputation, good software drivers and reliable support services. This whole audio recording thing is a very slippery slope. It requires some dedication beyond just plugging in a few devices and hitting record. Its fun though
I am a full time musican and have some feedback for you. Here is a list of trusted brands to choose from..
Capturing Sound from Casio Wk7500 through Behringer Xenyx 1204FX with UCA200 usb to rca interface into Garage Band on IPAD MINI First things first, I disconnect everything but the power cable to the Behringer Xenyx 1204FX, remember the programmers axiom KISS, keep it simple stupid. First, do NOT disable MIDi through Local Port in Casio WK7500 functions menu, in casio wk7500 Fnctions menu leave MIDI LocalCtrl set to ON (default). This allows wk7500 sound output through the T9 Line Out port, on the back of the wk7500. Connect a 1/4 inch y cable to the WK7500 T9 R/L Line Out port, to the input 1/4 inch port on a channel of the Behringer Xenyx 1204FX. I use channel 2, I suppose any channel will work, but I ain't messing with something that works, after a LOOOOONG time fighting it. Set the gain control on the channel (my channel 2) to about seventy five percent, 3/4s from full gain. Every other knob control of channel 2 is set at midrange. I enable the input for channel 2, the Mute push button is UP, tall. Every other channel is muted, the Mute push button is DOWN, short. I select the input of channel 2 to Solo, the Solo push button is Down, short. I have the channel 2 volume slider way up almost to red lining, to get enough volume voltage out through the UCA200 into the ipad mini and Garage Band. CD/Tape button is Down, short. Alt 3-4 button is Down, short. Main Mix button is UP, Tall. To offset the volume I have the phones volume knob turned way down to just a quarter, 1/4 of MAX. AUX Sends buttons are both UP, Tall. The Main MIX/Alt 3-4 button under AUX Returns is set UP, Tall, to Main MIX. The MODE button above the Main Mix sliders is raised, UP, Tall, set to Solo. I have the rca cables for the UCA 200 cross connected, that is, the Input to the UCA 200 is wired to the Output of the Xenyx 1204FX, Red to Red, right to right, white to white, left to left; and the Output of the UCA200 wired to the Input of the Xenyx 1204FX. The usb cable male plug of the UCA200 is plugged into the female usb port on a Genuine Apple Lightening to Full Sized USB 2go Female Port adapter. The real Apple adapters have a Smart Chip in them, so don't cheap out here. I have a known good PCI 1/8 inch trs stereo cable connecting the Ipad Mini earbud/earphone port to the T6 PCI stereo Audio In port on the Casio WK7500. This sends the output of the Ipad mini out to the speakers of the Wk7500, a much more satisfying listening experience, and it disables the mic of the Ipad Mini so you get the true input from the Wk7500 through the Xenyx 1204FX/UCA200 interface. Following a tip I got on the inet, I set the Ipad Mini to sleep mode before I plug in the fully connected lightening cable. From what I can see, the UCA200 connected to the Ipad Mini and the Xenyx 1204FX requires No powered usb hub. Play carefully with the volume controls, you don't want to blow up any equipment, especially your Ipad Mini. Garage Band in this configuration wants its volume slider way up high, in both the track dialogue and the Amplifier main dialogue slider. I have only recorded a solo track in GB in this config, but it Works. I also recorded with my tv in the room up way high, and GB picked up nary a peep from the tv or any other ambient sounds. I just got a crisp clear recording of the Wk7500. It lights, it runs, Garage Band hears the Wk7500. After confirming the setup, I switched my head phones from the Xenyx 1204FX headphone port to the Wk7500 headphone port, this mutes the Wk7500 speakers and sends all sounds from the Ipad Mini to the head phones, so's I doesn't keep the kiddies and the dog awake when I'm mixing at night. I had an initial issue with my ipad mini headphones port, that was solved by Apple support, by backing up the Ipad mini in itunes, and restoring the ipad mini and reloading my config, but that was quick and simple and paved the way for my Xenyx 1204/UCA200 to Ipad Mini setup, Thanks again Steve Jobs, myrip, and Apple Support. Happy, happy, happy. I have tried every ipad connector, every pc to ipad to keyboard config, two different music keyboards, and this is the only config I got to work.