First to even start making your own song you have to make sure that you have all hardware needed to record vocals. You need some kind of microphone, headphones, PC and that`s all you need for a start. Adding some mixer is great but let`s be real, a lot of you don`t have professional mixer at your home.
#1 Open Adobe Audition software, insert the instrumental at your first track. To insert instrumental all you need to do is to find instrumental you want to use in to folder and drag it to first track.
#2 "Arm" your second track, Arming means that you will record your vocals at on that track. You can do it by simply click on arm button.
#3 Hit the red record button and start sing, rap or whatever kind of music you making. pres space button when you finish.
#4 Now you have your raw vocal acapella and instrumental you previously inserted. Pres space button to hear your raw song and make sure you are satisfied how it sound.
#5 Now all you have to do is mess a little bit with the EQ on the left, and add some effect to your vocals.
That`s it. Pretty simple, right?
To know what effect to use please read this post :
Adobe Audition CS5.5 Mastering Tutorial