The maker of a Will must be of "sound and disposing mind" in order to have a Will drawn up for them. They must also not be under any pressure from outside sources or anyone who will stand to gain through that person's Will.
If someone is not of sound and disposing mind or is subjected to coersion or outside pressure from another person, then the Will may be contested.
Also, if the Will is not properly executed and properly witnessed. It takes at least one witness and its better to have two witnesses to your signing. If any of those things are missing, then you may contest the Will.
How Do I Contest a Will?
To contest a Will its usually best to hire a local lawyer who is familiar with the Surrogates Court and knows the clerks of the court well. It matters in the long run.
You must file a Petitioner to Contest the Will with the Surrogates Court and within the Petitioner you must outline the basis of your claims. At the very least, you must make out what's known as a "Prima facie" case that the Will was signed under duress or the maker of the Will did not have sufficient disposing mind to know what they were doing when they drafted and signed the Will.