How can low earning employees be confident and improve themselves in an organisation they work for?

How much you earn is not a measure of how much you're worth: it's just how much you happen to earn at the moment.  It's not even a measure of how much you are worth to the company - the CEO gets paid a lot more than the person who cleans the toilets used by the canteen staff, but if you had to pick one of them not turn up for work for three days, which would it be?

So there is no reason for a low earning employee not to be confident.  To gain confidence, and perhaps move to a better paid role, the steps are simple.  Do your best; strive to find ways to do your job better; study and learn; get to know everyone in the business and everything about the business; be enthusiastic; volunteer; and be pleasant to work with.