Hi, are you just "DI" into the mac and no pedals etc ?
If so you can run processors across that audio input, you'll get a bit of latency
so you'll have to set your audio buffer size down to 128 or 64 - remember to put
it back up to around 512 AFTER you finished recording to get more processing
on your whole song.
The Guitar amp pro and pedalboard plugins will work nice, use some compression and some overdrive to give you more "note" you can also spin
things on a bit with some delay if you like.
Plain 'ole DI guitar will die off quite soon if totally unprocessed.
PS _ remember to switch on "Audio input processing" in the Audio/Hardware setup page.
Seriously though, as MM said you need more than just the direct audio coming in from the guitar. Lots of cheap/free software that will help process your guitar a bit to give it some sustain, and the rest is all in your fingers my friend.