Is there a halachik obligation to write a will? If so, is there a specific way it needs to be written or can you just write one, sign it and make sure that someone else has a copy of it? Thank you.
There is no halachic requirement to write a will. the Torah has a default system put in place.
Were one to want to wish to bequeath certain assets to certain people, or assets one expects to acquire at a later stage in life, or to people not yet born, this can be done through a halachic will.
It will not suffice to write up your wishes and give a copy to someone you trust. A standard secular will, or the device created by a well-meaning person will rarely meet the halachic requirements. It is necessary to approach a knowledgeable Dayan who can interview you and find out your personal needs and prepare a document specific to you.
It is advisable to also have a secular will prepared in case the interests of the deceased are challenged by someone who might not accept the authority of a Beis Din. If there is anyone that might conceivably apply to secular courts to receive that which they believe is their share of the estate according to secular law, it is prudent to have a secular will.
Once you have a secular will, a knowledgeable rav can add a separate Hebrew document to empower the secular will according to halochoh. That way your interests as delineated in the secular will be enforceable in secular court and in Beis Din.