Answer One thing to remember is that the groups that you are probably listening to use huge powerful amplifiers. Also, without a doubt they are using different effects to create a very big sound. They could also be using 7 string guitars, or tuning 6 string guitar down. You will probably not be able to duplicate that sound exactly, without spending some cash on equipment. Sometimes I have students that do not even realize that there amp has distortion.
Distortion is key to getting a heavy metal sound.
Here's a way to turn on the distortion on a lot of different amps. You may also have a button that you have to push to switch between the clean sound and the distortion. Some very beginning amps do not have distortion. You could then buy an inexpensive distortion pedal to help you achieve a heavier sound. If your amp is a very small amp, it will have a small sound no matter what you do.