Put an envelope onto your audio track. A horizontal line will appear down the middle of the track.
Find a point where you want to change the volume of the track. Right click on the line at that point, then select "Insert Point" (or whatever it is exactly). A square will appear on the line.
You can then drag the square up or down to increase or decrease the volume at that point. You can also drag it left or right on the time line.
There are also video envelopes. I don't know what they do, and I think they only come with the full version of Vegas.
Don't forget to print out the manuals that come on your disc. They are BIG, and helpful. I duplex printed mine (on both sides of the sheet) on my laser printer, and had them bound at Office Depot.
Don't forget that the points on either side don't move - this means that you can have the volume fade up or down gradualy or you can make abrupt changes. It also means that you'll need 2 points for each change - one each to define the start and end volumes.
Thanks for the tips guys, I added a video and audio envelope and played around with it, I think the audio one will come in more handy for what I do.
(btw - Vegas Studio does come with video envelopes)
Also, I didn't print out my manuals, but did buy the "Instant Vegas Movie Studio" book by Spotted Eagle. I've found that pretty helpful getting started as well.