Clipping is the first thing to avoid when mixing vocals. Make sure you gradually lower the volume until you are unable to see any red lines along your track. The next step is to either EQ the background noise or completely remove it. The third step is to compress your vocals using Audacity's built-in compressor. The compressor ensures that all of your vocals are at the same volume level throughout the song and that distant vocals sound as close as loud ones. After compressing your vocals, you'd want to EQ your vocals by cutting out all unnecessary low-end frequencies and boosting high-end frequencies. The last step is to add a limiter, which acts as a powerful compressor and lowers the dynamic range of your vocals. You can always add effects like reverb to your vocals if you'd like, or just leave them dry.