For money management, just subtract your monthly expense(mandatory things like rent, insurance, phone etc) from your monthly income. Whatever leftover is your true monthly income/budget. From that budget, you have to decide what percentage of it can you put into your savings that you'll never touch except for emergencies.
As for credit, it's probably the #1 way of life killer in the world. When you hear someone is drowing in debt, most of time is from bank loans through ther credit. Do NOT fall for the trap of thinking bank loans is free money: it is not.
As
@jinri already said, the trick is to use a good percentage of your credit, and pay back all of it to build your credit. A good trick when your credit limit is low however, is to pay back
most of what you spend instead of all of it. The bank makes their money with interest payments and if you let some debt carry over, it'll let them now that not only will you pay them back their money, they can make some more money off of you with interest which can greatly increase your credit limit when the last term is over. Basically, with a low credit limit, leave 10 to 30% of what you owe unpaid and allow to carry over to the next month. Interest isn't an issue since the payment is so little due to your small credit score. My score increased 80+ points to 700+ in just 5 months that way when my credit limit was small.