There's a few factors you have to take into account:
- Your co-worker may have been there a lot longer than you, and may have a higher hourly wage for that reason.
- Maybe, as an unqualified laborer, you fall in a lower pay-grade than your co-worker
- Maybe, because you're still in training you were paid at a lower rate than what your normal hourly wage will be when you start the actual job.
Clearly, a lot of different scenarios could apply. The only surefire way of dispelling doubt is to ask your employer.
Call, or better yet, e-mail your boss, and casually ask what your hourly wage is:
Hello, I wanted to once again thank you for this employment opportunity. I'm very happy to starting in this new position come December 12. I simply wish to verify what the hourly wage I will be paid is, and whether there is any opportunity for over-time, or increased rates during weekends, or holidays. Regards, user44532
Showing a little bit of curiosity is perfectly normal, and innocent, as long as you're polite about it. The tricky part comes if you get a reply you don't like, such as "Hi, user 44532, your hourly rate is $14, and you can never get overtime!"
At that point you have to be careful how you handle the situation, as I'm assuming you don't want to either get fired, or walk away from the position. Furthermore, you already accepted the position, and I'm sorry to say this, but as an unskilled laborer you have very little bargaining power.
The best thing to do is to start the job, be absolutely brilliant at it for the first 4 months or so, and then speak again to your boss, and ask for a small raise:
Boss, I know I'm still new here, but I've been working very hard, and I believe I've shown great improvement. Would you be willing to consider a small raise? I really need the money.
Unfortunately, you shouldn't expect any miracles. The service industry is not well known for its generosity to the little guy, nor for its big salaries.
Your best bet would be to pick up some skills on the side, and buck for a promotion within the company.