A will is the only legal way to ensure that your money and possessions including property, is distributed to the people and causes that you care about in the manner that you have determined.
Without a will, your estate will be distributed according to the law, in a way that may not be in the best interest for you or your beneficiaries. E.g. Unmarried partners cannot inherit their partners estate.
Solicitors are regulated and therefore will ensure that your will is valid and meets your requirements. It is generally advisable to use a solicitor to ensure your wishes are going to be carried out.
Executors are the individuals assigned with the responsibility of carrying out your wishes and look to sort your estate. They will administrate the requirements of the will and fulfil the obligations outlined in the will.
A will can be stored anywhere, but should be stored in a place that is safe.
Should your circumstances change, it may be necessary to make alterations to the will. This cannot be done by making alterations directly onto a existing signed will, as this will render the document invalid. So it is advised to seek the assistance of a solicitor to amend an existing will.