Don't have a tube top to trace from? You can make a pattern from scratch.
Note: Depending on your knit fabric, you will have to take off about 1/4" for every measurement that you had to divide by four. This is because it will be looser in the body if the fabric is too stretchy. If the fabric isn't stretchy enough then you can add more. This is the most basic pattern and it's still a guess and check deal! Trial and error is the only way to learn!
Did you know SWN has a Pattern Making Channel?