Make an appointment with a solicitor, and before you see them, make a list of all your assets you can. Have a think about what you want to happen to things - the house, the main money, the valuable items around the house, the business, sentimental items, photographs and certificates and so on.
The solicitor will talk to you about things, and bring up things that might not have occurred to you, and draw up a draft will to send to you. When you read it through you will probably notice things that you would change, or add, or people you want to leave things to who you missed out the first time.
You will need one or more executor. The solicitor will act as executor but will, as ever, charge for that service, the amount depending on how long it took them to carry out your wishes. If it is a member of the family or a friend, check that they will be willing. Don't pick someone who is a lot older and likely to die before you do.
The solitor will change things ad infinitum until you are ready to sign, but, of course, the final bill will reflect how much time they spent on it, so the more you have thought about it first the better the price.
When you have signed it, put it away somewhere and tell the family where it is and who is to be executor. Don't hide it up the chimney for safety to keep its whereabouts a secret!