Hello, as many others, I am a beginner and learning a lot about singing in the 30 day singing course. I'd like to know the following:

Is the melody of a song always sung in chest voice?

I'm most comfortable singing in head voice, and like to work on singing in chest voice. What are some tips that will help in accomplishing this goal?  

How does one know if they're an alto, soprano, etc? 

Great questions!

1. Not always! And I'd even say not usually. Most melodies have a wide enough range that they require more than one vocal register. This is true for most pop, rock, jazz, and r&b I can think of. One exception is country music by male singers: there are many that tend to use chest voice exclusively. If you want to sing a song using just one register, you'd need to find one with a limited range. Here's a useful site that allows you to search for songs within a defined range: http://www.myvocalrange.com/.

2. Start with your speaking voice! That's the foundation of your singing voice, and it's very similar in tone and function to your chest register. I'd have you speak the word "hey" or "no" on a pitch, then begin singing (sustaining that pitch) with a singing exercise. Something like: "hey (spoken, A), heyyyyy (sung, A-B-C#-B-A)". 

3. Here's an article in which I walk you through the process of finding your range: https://www.30daysinger.com/blog/finding-your-vocal-range. 

Best of luck!