Look at keyframes in the timeline audio track. But first, why not take some time out to read the user manual. You may find it actually tells you how to use Premiere Pro.
There are a couple of effects under audio. The one I use most is constant power. Drag it to the clip and adjust to taste. For more control use keyframes on the volume.
If you are unsure of these things, either check YouTube for free video tutorials or subscribe to Lynda.com for a month (though I have to admit the Premiere Pro tutorial on there is not as good as others I've watched). There are also lots of free videos on the AdobeTV site so be sure to check those out too.
Then... if in doubt, get the manual out
There are several ways to adjust Audio in PrPro. The method used will depend on exactly what one wants to do.
This ARTICLE will go into more detail. I agree with J-MS, that Keyframing the Fixed Effect>Volume is likely the best for you. This will be Clip Keyframing. One can also use the Audio Mixer to automate Track Keyframing, which will do the same thing, but differently. That article tells you the differences.
For tips on Keyframing (almost any Effect, Fixed or added), I recommend the Help File, as it covers Keyframing in great detail.
Good luck, and hope that helps,