How can you determine atomic mass?
I would suggest you to find the periodic table. If you can't find one at your school just go to google and type "Atomic mass in the periodic table". Then you will see different style of them and some even explain the table to you. So, when you find the one you think you might like, copy the picture and save it on your computer. After that, print it out so that you can still have it with you or put it in your binder.
I hope that helps!