Any person who is over 18 can sign as a witness. However, the witnesses should not be people who benefit from the will (or the husband or wife of one of these people). If you witness a will and you are named as someone who will benefit from it, you will lose your right to that benefit when the person dies.
Special rules apply in certain situations, for example, if a blind person or someone who can't read or write wants to make a will, you should speak to an adviser to find out more about this. See 'Further help' for details of where you may be able to find an adviser.