Your will says who you want your money and everything you own to go to when you die. If you don't make one, the law says who will get it, which will be your closest relatives, and that might not be what you want. Certainly if you have a partner you're not married to, they will get nothing because they're not a relative. So make a will and you can say exactly what you DO want.
It's important to get it right because nobody can ask you what you meant when you're dead - and that's the only time anyone will want to read it!
As you asked the question, obviously you don't know how to do it, so the best advice is to find a solicitor and pay them to write it. First decide how you want to divide everything up and who should get specific things or amounts of money, You could also leave money to a favourite charity. You also need to decide on an executor (preferably two) who will have to follow your instructions when the time comes. Now see a solicitor with your notes and you won't waste time with him or her - their time costs money so do your thinking first in your own time! Then they can write it up in the right kind of language and get it signed and witnesses - your signature must be witnessed by two people who also sign it or it's not a valid will.