Here are a few tips that might help. Your head voice is the high range sung from the head. Your chest voice is sung from the chest. It is mid to low range. If you put your hand on your breastbone you can feel the vibrations. Try singing a high note and gradually making it lower or vice versa. There comes a point when you suddenly feel the vibrations in your chest that is the start of your chest voice. That is the area you need to work on. It may take a while to find the vibrations and develop the feeling.
Vocal training can help most singers but is not always necessary. Some people are naturally gifted and can sing to a good standard without any coaching and may want to develop their own style. As was pointed out there is a danger that you can damage your vocal cords singing incorrectly. This usually comes with overstraining eg. trying to sing too lowdly or notes your range cannot reach. Most folk know and feel when they are straining it usually comes with a slight burning sensation in the back of the throat. Do not continue to sing in this fashion. A good tip is to record yourself singing different types of songs. Play it back and it should be clear as to what areas are your strengths and weakness. Practice to improve the weak areas.