If you're getting a lot of noise, turn the gain down and the channel volume up. You'll use the channel volume knobs to control your mix along with the "Main" volume pot. Channel volumes will control only that channel strip, and the "Main" volume is your master output that controls the volume going to the computer. It's OK to leave a lot of headroom on the gain knob. It lets you hit the track later with more effects without clipping.
Anyway, you are about to get into the wonderful world of "routing." It is easier to understand if you imagine each part of your mixer as a separate bit of hardware. So your mixer might have the channel strips in one part, the effects processor in another, the main outputs in another, and the monitor outputs in another. For each box there are two cables. One cable sends audio to it, and the other receives audio from it. You can switch the cabling around to various parts of the mixer before they end up at the mains and monitors.
So for example the simplest configuration has the mixing channels connected to the main outputs. You have another box called the "effects" box that you can send audio to directly from individual channel strips and have the sent audio (called the "wet" signal) merged back into the unprocessed audio (the "dry" signal) before it hits the mains and your ears.
So lets do this the same way.
On your mixer, select the effect you want to use.
Turn the FX Return Level all the way up.
On the channels you want to use effects on, turn the FX Post Send dial all the way down.
Press the "Main to Monitor" button.
Make some noise for your microphone and slowly turn the FX Post Send dial up.
You should hear the signal become more and more changed as you "send" more audio to the effects processor.
So in this case, the FX Post Send knob controls how much signal will go to the FX processor, and the "FX Return Level" controls how much comes back. You can dial them up or down to taste. The "Main to Monitor" button just connects the mains to your monitoring headphones, which are separately routed.
This allows you to use the effects on multiple tracks. For example if you wanted a lot of reverb on the guitar and just a touch on your vocals, you could do that by "sending" a lot of the guitar track and a little of the vocal track.
When you hear people talking about "send effects," this is what they mean. You are "sending" some of the audio to somewhere else, and merging that return signal back into the mix.
I also just realized that when I am talking about clipping signals, I mean the red light on the channel strip itself. That is the one that shouldn't come on. The signal meter lights on the right side of the mixer should be showing activity. That is the signal going to the mains. My bad for not catching that.
Someday we'll win this thing...