Who can file a lawsuit?
Under tort laws and civil laws in the United States, a person must have "standing to sue." This means that he must have suffered a legal wrong that the law can provide compensation for.
One requirement of standing to sue, in order to file a lawsuit, is that the party suing must have been wronged in some manner by some legal violation. For example, you may feel wronged if your neighbor refuses to say hello to you, but this is not a legal wrong. A legal wrong occurs when a legal duty is breached. You may file a lawsuit against your neighbor if he drives a tractor into your house, for example, because he has breached a duty of care and come onto your property and caused damage.
You may file a lawsuit for car accidents, as all drivers owe each other a duty of care. You may also sue for medical malpractice if a doctor provides substandard care below his professional duty; for a breach of contract if a party doesn't comply with his duties under the contract; or for civil rights violations if a company or other entity infringes upon its duty to respect your legal rights. The wrong you suffer doesn't have to cost you money, in other words, it can be any legal wrong. It is also possible to file a lawsuit in some cases even if you were not wronged personally. For example, if a doctor engages in malpractice and it results in the death of your husband, you can sue for his wrongful death.
Another requirement for "standing to sue" is that you must be seeking a legal remedy. If someone did something wrong but you didn't actually suffer any loss, there's no real reason to file a lawsuit and you aren't going to get much if you do. You must sue in order to get something, either money damages or an injunction or a command that the defendant do something.
There may also be a myriad of other requirements for "standing to sue," including personal and subject matter jurisdiction and statute of limitations. If you are considering filing a lawsuit, consult with an experienced attorney for help during the process.