You change the iTunes volume setting using the Volume control in the upper-left section of the iTunes window. The maximum volume is limited by how loud your computer's volume is set (you change the computer's volume in Sound pane of System Preferences).
If a song plays more loudly or quietly than others, it may have been converted to MP3 using different settings. You can also adjust the volume of an individual song.
Click a song to select it, then choose Get Info from the File menu. Click the Options tab, then drag the Volume Adjustment slider to change the song's volume.