Hi Smithsk89.
When you choose the input for the track the you are trying to record on you will be given 3 options for each if the stereo inputs of your sound card. They will be listed as something like "Input 1&2 Left, Input 1&2 Right, Input 1&2 Stereo" for each stereo pair. If you plug your guitar into the first input on your interface for example, you need to choose "Input 1&2, Left" This will give you a centered, mono input that will pan correctly. If you use the second input, you would choose "input 1&2, right". If you have a stereo source such as a keyboard, you would choose "Input 1&2, Stereo".
Your interface may also feature zero latency monitoring using its own mixing utility independent of Sonar, but we can't help you with that without more system details.