I'm assuming the "platinum" sound card you're refering to is a Sound Blaster Audigy Platinum. Sound Blaster cards are not ideal for recording audio, although they are not the worst in the world either. You should be able to get some decent recordings out of your current setup depending on what your PC's specs are.
1. Take the RCA cables from your mixer's monitor out and run the cable into the sound cards line in inputs. They may not be clearly labeled on the card itself but if you have the paperwork it came from it will tell you which is which. You will most likely need a "Y" adapter that takes left and right female rca connector into a male stero 1/8" connector to go into the card. Make sure that your monitor output from the mixer is working by running the cords into something with speakers.
2. I don't know what software handles your card in Windows, but if you go to Control Panel you will find an entry for your Soundcard's applet (i.e. Mine shows up as an applet in control panel called "M-Audio Delta". Yours may show up as "Creative Audigy" or something like that). If there is no specific applet then just open up Windows' sound mixer which can be accessed by opening the "sounds and audio devices" entry in Windows control Panel, choosing the "audio" tab, and in the sound playback section, choosing the "Volume" button. This will open the mixer applet.
You'll see the mixer, and you should see a channel there for your line in. Make sure it is unmuted if you want to hear the audio coming in to the computer for your speakers (there should be a simlilar set of controls in any custom applet made by your manufacturer).
Now choose "Options" at the top and click "properties". Select "the "recording" radial button and click "OK". Now you will have a set of new mixer channels for recording. You will need to enable your line in channel (again, a custom applet should have some settings similar to this or be much simpler altogether).
NOTE: All manufacturer's vary in terms of how they deal with controlling their audio interfaces through your system. I have 2 M-Audio interfaces, one is the Audiophile 2496, a PCI (internal) card and the other is an Omnistudio USB, Each one has it's own applet that runs in my system tray, and the Audiphile's is complicated with many options because it has no physical controls, while the Omnistudio's is extremely simple, letting me choose only bit depth, number of ins and outs, sample rate, and buffers (We'll get to these settings in a bit...they can be controlled by the recording app or in your applet, but for your setup the defaults will probably work). Whatever options you have, you will need to enable your line in some way.
3. Once this stuff is done, you will need to go into Sonar and set up the Audio there. I have limited experience with sonar, but I know that in it's preferences, you will be abe to choose your device, your bit depth, sample rate, and buffers too. Once that is done, you create an Audio track, select your soundcard's line ins as it's input, arm it for recording and hit record.
For more specifics on Sonar, you will need to refer to sonar's help or customer support.
I realize there is a lot of info here. There are simpler programs available to use but it would involve you spending more money. Honestly, once you get your mixer hooked into the right input and putting sound into it, learning to record the simple stuff you want to shouldn't take long at all.
If you are still having trouble getting sound into your pc, see if you can find a local music gearhead to come down and check out your setup. They usually hang out in guitar stores or pro audio stores and may be willing to volunteer given you are a community center giving local musicians a venue to play in. I would go so far as to call the managers of local guitar/music stores asking for volunteer help to get going.
GOOD LUCK