If your guitar takes a battery, it has a built-in preamp and
you can plug it right into a line input.

A guitar with passive pickups (no battery) generally needs an
input preamp.  You can always try right into the iMic, but
most likely the level will be quite low.  Be sure to go into
System Prefs / audio and find the horizontal "meter" with
grey dots.  Set up for 16-bit, 44.1kHz mode, and play your
guitar as loud as you ever will.  Adjust the input level so
the grey dots on the meter go nearly all the way to the right,
if you can.  If you can, you're golden.  If they don't go much
past the left (low) end, then you'll want an instrument preamp.

"Instrument preamp" sounds fancy, but a lot of things will
work fine.  For example, if you have an old cassette recording
deck with mike inputs or 1/4" jacks, chances are you can use
that.  If you have a small mixer with 1/4" jacks and a "pad",
"trim", or "gain" control that affects this input, that should
work -- put it between the guitar and iMic.