The conditions under which you can come to the UK, the length of your stay etc, depend first of all on your country of origin. Without knowing that, I can only give you a general answer, but hope some of the following may be helpful:
If you click here you will find information about the criteria for a general visitor. If you fulful these, you can click here where you will find information about the documentation you need. As you will see from this page, your fiancee can't really sponsor you. She can help you with your application but she wouldn't count as a guarantee. You have to provide the documents and proofs yourself - if you click on the right side of this page you can download a list of supporting documents needed. As you will see, on of these documents can be a letter from your fiancee, but if you are only planning a general visit then it is enough to say that you are visiting her.