The Samson G Track is both a microphone and audio interface. You would select that as both recording device and playback device. Connect your monitor speakers or headphones to the Samson.
Some folks have had success downloading the ASIO4All audio driver to use with the G Track.
But it's not the best device to use with your DAW. An interface with true ASIO drivers will almost certainly give you better performance.
Look at the signatures of many of the users here to see what audio interfaces are used. You'll see lots of Focusrite (what I use), M-Audio, MOTU, Presonus, Cakewalk/Roland, some TASCAM and Alesis devices. I recommend Focusrite. My Saffire has been rock solid for years and it works great.
If you decide to try ASIO4All first and later want to get a different interface, you'll have to delete ASIO4All from your computer before you can use the new device's true ASIO drivers. 
 

When shopping for an audio interface what is most important is your needs for connectivity and monitoring. 
To many people run out a grab an interface just because everyone else recommend it only to find they now have to buy some more gear to complete their set up. 
Make a wish list of what you really need in the way of connections and monitoring and start shopping. 
I just bought a Scarlett 6i6 because it had exactly what I need. I wrote a review: 
 
http://www.gearslutz.com/...rite-scarlett-6i6.html

So to get this straight; I choose a decent audio interface. I plug in my Samson Mike to it, and also my guitar/piano whatever, and then tell my sonar software to point to the audio interface rather than the Samson mike as I do at the moment?

Yes and No, the trouble is the Samson mike is not XLR so cannot be used with an audio interface. 
I was speculating a while ago that someone would eventually make a converter box. 
But the rest you are correct. You set the audio interface as your in/out with Sonar and all your audio programs. 
You'll have to buy a mike that has a standard connection ( XLR) Happens a lot. 
This is why I was saying it's a bit of a rip off that people purchase these mikes, they don't work as advertised, and they end up having to buy a normal mike and a audio interface.`I would sell the Sampson to try and re coup your money. Or take it back to the store and see if they'll trade it in on an interface.  

Cheers,
At the risk of an obvious question; are the advantages of the audio interface in terms of the quality compared to the usb mike  or just ease of use, or both of these things? Remember that I just want to get reasonable results, not pro ones(although of course that latter would be nice to achieve!)
 
And a decent mike would be?
 
Rich

A audio interface with good pre amps like the Scarlett series, and any good mike will give you all you need. It's tooo bad you can't use the Sampson. It will cost you another $100-$200 to replace it. 
Your mike? Without comparing it to other mikes I cannot say but my guess is it sounds just fine. There are many, many mikes on the market, too many. And depending on what your looking for, most will sound perfectly fine if your level of expectations is in the lower end of the scale. Nothing wrong with that, this is just a hobby for most of us.
 The good old SM 57. If I owned just one mike, this would be it. A Beta 57 might be better. 
For my Vocals I'm using a old, old Audio Technica mike that looks like a 58. The number wore off 20 years ago. But I like what my voice sounds like both live and in the studio with this mike. It was only $120 back when. So finding your favorite mike is a personal thing. If you can try as many as possible. 
 
It's worth a mention that a work around for you is to not use Sonar at all. Sonar is the worlds pickiest DAW when it comes to drivers. Your mike will work fine with programs like Audacity. 
 
What some folks do is record into Audacity and import those tracks into Sonar to mix. Clunky and not perfect, but FREE> 

I just Goggled  " converting a USB mike to XLR"  
The results are what I expected. It does not exsist.  And advice from all is " If you want a good mike, don't buy a USB mike, they are not for audio but podcasting( talking) " 
 
I was wondering if there was a way to just re-wire one and by pass the interface part.. nope, at least not worth it to the average person. Sell it... 
 

The audio/interface takes the place of a regular computer type soundcard (or built-in sound) on the computer. As with all things, Professional is a word that can be tossed around. There are better audio/interfaces that can cost thousands of dollars and there are other ones that can cost a couple of hundred or even cheaper ones that can cost $50. Usually the couple of hundred dollars one will have inputs for guitar, synth, microphone, whatever, and also Midi in and out. Cheaper ones probably will not have midi at all included, and with all things, then there is the quality of the device in the end and what you want it to sound like. As with anything, checking out reviews and maybe over at Gearslutz forum is a way to find out with your budget how much you want to spend and how you want to use it.
 

As I wrote above, the G-Track is an audio interface. If you don't want to spend more money or can't afford to spend more money at this time, download ASIO4All and give that a try. It has worked for many folks and saved them some money, too.
It may work, and may give you not-pro results.
For the record, it didn't work for me when I tried it in my Sound Blaster Extigy sound card days.
 

Lots of ideas..thanks I'll go away and think...and think some more!