I'm gonna assume you're going to record electric guitar by the sound of your inquiry, and that you don't have an audio interface (kasi may libreng mas magandang recording software ang interface na bundled).
1. if you're not planning to spend north of 5k for a mic, forget about recording your amp or acoustic guitar.
2. if you're going to record straight into the mic/line in of your computer, there are two scenarios
2a. if you have effects AND an amp sim (Sansamp, multi effects, Artec Twinhead, etc) go ahead, just jack it in.
2b. if you don't have effects and/or an amp sim, ergo you're recording a dry signal for plugins to process, abandon Audacity altogether and get REAPER. Audacity doesn't do real time filters, unless you have the utmost patience to record dry guitar signals, then undo, reset, and apply settings to your track.