Invest what you save. Sounds simple enough. The problem is in the actual doing of it on a day to day basis. Many are spending more than they earn, in hopes that one day they will get a job that will pay them more than they are spending. The trouble is, we tend to always spend as much or more than what we are making.
Credit cards are a financial trap.
They make it easy to spend more than you make. Not to pay the full balance each month is not wise. Once you have some money in the savings account, there will be a temptation to spend it on things that you don't really need. It would be much wiser to keep the money in savings for those unpredictable emergencies. That way, you do not have to go in debt when the air conditioner goes out.