When I had that much gear, I just programmed the drums through MIDI (yes, you click the notes on a grid, like a piano roll), and had all my electric instruments run through the Mic in using a 1/4 to 1/8 adapter. The vocals ran in using a CDR King mic (yes). It looked like this:
Throat > CDR King mic > Mic In > (Autotune/ gSnap) > DAW (this would be Garageband for you)
Electric guitar/bass > (pedalboard) > 1/4" to 1/8" adapter > Mic In > amp sim (I use Guitar Rig 5 or Shred 2.0, I don't like the built-in one) > DAW
Mouse clicks > software drums (I use Independence Free, the one in Garageband may be fine) > DAW
Keyboards/iPad > Mic In > DAW
If you're good with a muddy demo record, you can actually stop reading now. But please do read on.
When I got some cash, I bought this now kinda hard to find gear called M-Audio M-Track, which has two XLR(mic)/TS(guitar) simultaneous inputs, as well as MIDI in and out, phantom power (for condensers), headphone and monitor outs. All for around P5,800. A comparable product without MIDI I/O would cost around 9-12K today (MBox comes to mind). You can plug in two mics and/or guitars simultaneously with this one, and if using your digital drums, you can plug it in using a MIDI cable you can get for a couple hundred bucks. You'd still need to use amp sims for the guitar signals, but at least you won't need the preamps.
I used to use an Avid Vocal Studio, which is a USB only condenser mic, worth around P6,400 I think. Don't do that for now. If you have an interface like the M-Track with XLR inputs, invest in either an Audiotechnica AT2020 condenser mic (great for vocals and acoustic instruments, P4K++) or an Audix i5/Shure SM57 (it will record everything, P5K++).
The gear may seem rather expensive for just demos, but these stuff will last you until you're rich enough to have your rough song sketches recorded by professionals every time you please. Especially the SM57/i5. Those will record everything and they last pretty much forever.
Do the math too: single purpose USB converters you can only use one at a time cost P2,000 each, and you'd have to buy two (guitar to USB and MIDI to USB), and still they'd last two years tops, no resale value. And you still have to buy the mics.