I was wondering if spending more (50-89%) or less (1-25%) of credit limit increases or decreases credit score depending on the amount spent on the credit card. I have recently turned 18 and became employed.
Spending a reasonable amount (<30% of the limit), and paying it fully off every month, will certainly help your credit score.
However, there are several large components that constitute credit scores, and the other factors are probably much more significant: payment history (so make sure to pay your bills, rent, etc. on time, every time); average age of accounts (so don't close your oldest accounts), bankrupties (don't have any, of course), and some more.
If you keep those in mind, your credit score will increase over time, and your credit limit will increase too. Remember that a high credit score means that your payment history shows that you are a good debtor, and pay as agreed. There is no shortcut to a good history- it takes time and consistency.