You need sufficient income (usually above 18K)
The new regulations that went into effect a couple of years ago require that people under 21 must provide proof of sufficient income to qualify for a credit card. You also need a parental co signer.
Most companies will want to se a STEADY job record of at least 12-18 month prior to the application, plus sufficient income, usually above 18K. In most cases a part time job (freelancing probably included) won’t suffice.
What card have you been trying to get? You stand no chance with a regular credit card. You may qualify for a secured credit card. They are easier to get. Try Capital One.
Since your parents help you, why not ask them to put you as an authorized user on their card? It’s called credit piggybacking, and it’s the best way to build good credit. See credit-piggybacking.html for more information.
BTW, do you have student loans? If you do – they build credit for you as well.
Good luck.