Assuming you are running Garage Band '09, select the track that contains the wave file you just added by clicking on its icon on the far left and then click the information button on the bottom right. The right panel will change to show information about that track. Click the Edit button in the tab and you will see a list of effects applied to the track already. There might not be any but there should be a few blank rectangles. If hover over one, you will see it display "click here to add an effect". Click and choose AUBandPass. You can then adjust the filter center and width by clicking on the icon and using the graphic display.
But to be honest, that isn't quite the right filter. You should probably try AUHighShelf or AULowPass. Both of these will allow you to remove just the high frequencies you want while leaving the lows intact. Bandpass removes both lows and highs at once. HighShelf will allow you to remove a variable amount of high frequency content while LowPass will be more drastic.
Keep adjusting until it sounds good. Then click Share and export itunes or burn a CD.