Please edit the sentences below if you can

I think that cellphones are useful for many people because we can talk to many people all over the world thanks to them. These days, we can also use the Internet via cellphones. Although there are such good points, there are some problems. For example, if we leave a cellphone somewhere, some one will pick it up. He may be a bad person. If he uses it badly, we may be involved in a trouble. Another problem is that is costs much money to use cellphones. These days, we can’t live without one. So we have to pay attentions when we use it. Cellphones are useful if we use them carefully, so we should always think that there might be a problem. (122)