Congratulations on landing your first job. Your question brought to mind just how useful a copy of the Catechism of the Catholic Church is in our everyday lives. I highly reccommend that you purchase one with your first paycheck so you will be able to feel more confident about living your faith. In short, it is not a mortal or even a venial sin to work on Sunday if it is a prerequisite for your having the job. All that is necessary to complete your Catholic obligation in regard to the 3rd Commandment is that you get to Mass which you can do easily on either Saturday afternoon, evening or Sunday at some time of the day. We are fortunate that there is almost always a Mass somewhere at a convenient time to get to Mass without interfering with our employment. Sometimes, you may need to ask your employer to work around your obligaiton, but those instances would be quite rare in general. The CCC will be a most valuable aid in living your faith and can be purchased in paperback for less than $10. It’s a great reference book.