The main way is to sell things to Timmy and Tommy. You can sell just about anything. The easiest would be to catch bugs, fish, etc, and sell them. But if you have furniture, clothes, etc. that you don't want, you can also sell them. If you don't want to work, you can buy Animal Crossing Bells directly on the ACItems website.
Sometimes, balloons flying over your island will contain bells. Shoot them down with a slingshot and grab the present. As I said, they only contain bells sometimes, so you may get something else instead.
As long as you are diligent enough, you will get more ACNH bells than others. If you are a casual player and don't have time to work, then you can go directly to the ACItems website to Buy Animal Crossing Bells, and we will serve you online 24 hours a day.