I work for a credit union as a loan originator. This means I spend my day looking at the credit reports & incomes of people applying for new loans, and explaining how our loans & credit in general work.
Credit cards only report your statement balance (the balance you see on your statement, from the end of the billing cycle), your credit limit, and if you are 30 days + past due to the credit bureaus, not if you are carrying your balance from month to month & paying interest on it, or what your midcycle balances are.
Some lenders won't report an update on your card if you have no balance and no activity, so do try to use the card at least once a month if you're paying the balance off in full.
If your credit limit is only $300, you should try not to charge more than $150 in any given month. People who max out their credit cards are more likely to file bankruptcy, so using a high percentage of your limit is bad for your credit score.
As mentioned by No Name Guy, myfico.com is a great source for credit education - it's where the Fair Issac Corporation posts what's publicly available about how your FICO credit score is calculated.