If I got fired, what should I say at an interview? ?

You said that you'intentionally' tried to switch a fake note, so this of course is fraud and you would have been dismissed for gross misconduct. Moving on, you have some choices.Where this matter will come to light is if a new employer asks for a reference, and this is on your HR record, the old employer may be required to reveal the information. You can ask your old employer for a copy of your HR record, and also ask for a copy of any reference that is sent. If you were not at the job for long, you could leave it off your resume, this will leave you with no reference contact details but you could give another reference eg. school/college, voluntary work and a character reference from someone you know who is not family. This would give you a fresh start- so in the meantime, while looking for paid work, do some voluntary work with a charity in your area to get a new reference. I've been fired myself in the past, it happens to most people at some point so don't panic, you made a mistake and have learned from it.Then at the interview, you will have interesting and positive things to talk about instead of getting hung up on your mistake. Good luck!

say nothing about the first job and do not make  the same   mistake again.

Usually stores have some way to tell if a large bill is fake or they usually have scanners, so you can tell if a larger bill is fake. If they didn't train you on how to tell if a bill is fake, you should be educating your self. If your store has scanners, you should be scanning (or trying to tell if a large bill is fake) $50 bills and $100 bills. If you notice it's fake then don't give it to a customer. You got caught using counterfeit money. Unless you were short changing a customer it's not exactly stealing, but using fake money isn't legal. If you weren't on the job for more then a month, don't bother using it.

Honesty is the best policy.  You attempted to steal $50, but you were caught, so you weren't actually able to steal it. 

You keep making "mistakes" like that and your job future is very bleak. Good luck.

"I have accepted my mistake and trying to learn from it"- Yea right! You were caught with your hands in the cookie jar, lets not act like you chose to do the right thing and confess to illegal acts. Trying to learn from it? There is no trying, it's simple you apologise to your old employer for stealing, and you NEVER do it again, there is nothing "to try" here, you just do it.

If you were smart? You wouldn't put even put it on your resume.

Don't bring it up. Chances are they won't contact them or if they do, they won't mention it.

Do you even need a resume for the kind of jobs you will get?

Incredibly stupid but nobody has to know. Lucky you did not go to jail.

If you worked there more than 3 months, you should have the job included on your resume / application.

What should you tell employers?  I didn't understand company policy and have learned from my mistake.

However, if you apply for retail positions, be mindful future employers might participate in the retail employers theft database, which will show that you attempted to steal from the employer.

Do not put that job on your resume. Do not tell anyone about what happened. Do not repeat it to future work colleagues. It will backfire on you. 

Do not put the job on your resume and NEVER speak of it at work or to your coworkers.

Consider yourself very, very lucky if your former employer didn't call the police.   Counterfeit (fraud) is a very, very serious federal offense compared to petty theft.    You could have ended up spending 15 years in prison.  I'm seriously not kidding.