The first thing I was taught on my first day at my very first job was how to count change. My boss emphasized that every single customer who paid in cash had to have their change counted back to them. Of course, being a conscientious employee I did so for all of the four years I worked there, and I can't tell you how many people complimented me on being able to do so. The supermarket in our town didn't teach its cashiers to count change, so our small grocery store stood out because of it. And of course, when we had power outages which could last for hours we could carry on as usual, tallying up the customers totals by hand with pencil and paper, and counting the change like we always did.
Our customers appreciated it, because it reassured them that they were being given the correct change and not trying to rip them off. Our till was also an old one, which displayed totals but not the change due, so counting change was the easiest way to double check our own counting. Leaning to count change made me more confident with handling money, and faster at counting out change when the time came to count out the till at the end of the day.
Unfortunately, counting change seems to be a dying skill, with fewer and fewer people knowing how to do it. This seems like a shame to me, because even if you rarely have to use it, simply learning the skill can improve your confidence with basic arithmetic and with cash handling. As a bonus, once you've learned how to count change properly, you'll immediately be able to sork out if you've been short changed in a shop!
As for how to actually count change, simply start counting with the lowest value coin, and place the coins and notes one at a time in the customer hand. (One thing I often had customers say to me was that they liked having change counted rather than being "dumped" into their hand.)
Say a customer spends $14.15 and gives you a twenty dollar bill. Say "That's $14.15" hand them a dime, "that makes it $14.25", then three quarters, "makes "$15.00" and finally a five dollar note "and five makes it twenty dollars." I have demonstrated with American currency, but the same principle applies, matter which denomination coins you have.
Of course, once you become practiced at counting change you'll be able to do it very quickly and still maintain your accuracy. Even if your job doesn't require you to do it, if you have time where you work, many customers do appreciate having their change counted back to them.