Too much volume control

Faders are your first vol control.  The sound comes in and the faders are at unity (or 0 dBs - adding and subtracting nothing).  The first thing I do is put up a rough mix by ear, adjusting the vol via faders.  Actually, I usually mix a bit while composing to get a sense of the song.  The alternate methods of vol control, like gain (trim) or make-up gain in comps, come after the vol and automation of same.
 
External synths are usually set to 75% or so of the maximum - I've always found that is close to the optimum for gainstaging both for internally for the synth and for input - that gives you gain to add w/ the DAW fader or to come down from.  Most softsynths are pretty much too loud when first open them, so if the vol knob is easily accessed I'll use that or adjust the vol of the DAW fader.
 
The monitoring system is really one you have to learn - there are no rules unless you set up for the K-system, and it is pretty specific and came out of film.  Although there is no average vol level to set your monitors to, although there are endstops to what works.  Too loud and you can't set track levels properly for quieter systems and the vocals and leads will be too low.  Set the monitors too soft and they'll be too loud at louder listening levels.  Too loud and you'll burn out your ears faster during a session.  Too soft and you'll burn out trying to concentrate and hear into the music.  It is a judgment call from both experience and "your personal style."
 
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Faders are your first vol control.  The sound comes in and the faders are at unity (or 0 dBs - adding and subtracting nothing).  The first thing I do is put up a rough mix by ear, adjusting the vol via faders.  Actually, I usually mix a bit while composing to get a sense of the song.  The alternate methods of vol control, like gain (trim) or make-up gain in comps, come after the vol and automation of same.
 
External synths are usually set to 75% or so of the maximum - I've always found that is close to the optimum for gainstaging both for internally for the synth and for input - that gives you gain to add w/ the DAW fader or to come down from.  Most softsynths are pretty much too loud when first open them, so if the vol knob is easily accessed I'll use that or adjust the vol of the DAW fader.
 
The monitoring system is really one you have to learn - there are no rules unless you set up for the K-system, and it is pretty specific and came out of film.  Although there is no average vol level to set your monitors to, although there are endstops to what works.  Too loud and you can't set track levels properly for quieter systems and the vocals and leads will be too low.  Set the monitors too soft and they'll be too loud at louder listening levels.  Too loud and you'll burn out your ears faster during a session.  Too soft and you'll burn out trying to concentrate and hear into the music.  It is a judgment call from both experience and "your personal style."
 
@

Actually the signal first goes through the input gain/trim control at the top of the channel strip
 
To the OP - please be aware that NONE of the controls inside Sonar can affect your levels when recording. You can only set input levels outside of Sonar before the signal hits your converters. If it's too hot coming in you will clip  the signal and you'll have ruined your recording.
 
The usual place to adjust your input levels is at your interface.
 
Once you've got a a decent level coming in (Goldilocks level, not too hot, not too cold - anywhere between -6 & -15dB is fine) you'll find when you come to mix things will be a whole lot easier.
 
This is know as Gain Staging - you can read loads about this, here's a very good starter:
 
http://www.soundonsound.com/sos/sep13/articles/level-headed.htm