Highlight the portion of the song you wish to modify. Select the entire track if you wish to modify some aspect of the whole song.
Select the desired effect from the "Effect" drop-down menu.
Use the "Noise Removal" effect if you wish to eliminate all of a certain sound from the track. The Noise Removal effect will ask you to highlight a sample containing only noise, from which it creates a noise profile. Then, you can remove audio matching that noise profile from the selected region.
Use "Equalization" to increase or decrease the relative volume of sounds at a certain frequency. Audacity's Equalization effect allows you to add points at whatever frequency you wish and adjust the volume accordingly. Furthermore, there are a number of predefined equalization settings, if you are searching for a specific sound.
Use "Amplify" or "Normalize" to adjust the overall volume of your song.
Use the "Compressor" to alter the distance between the maximum and minimum levels of your song. Compression can boost the overall level of your audio while decreasing the volume of whatever parts of the song surpass the specified threshold level.
Apply a "Fade In" and "Fade Out" to the beginning and end of your song, respectively.