Some people are planners and like to plot out their novels from start to finish, before they write a word. Some are pantsers, writing into the blue without any plan, flying by the seat of their pants, and most of us will be somewhere in between. It doesn't matter. What does matter is that you need to start is an idea - but not just any idea. You need a story idea.
A story idea suggests future developments. So, you have an idea about a group of friends at a wedding - I choose this because this is the idea I started off with. The trouble is, a group of friends at a wedding doesn't suggest anything much in terms of story.
You need to put in some conflict. A group of friends at a wedding, one of whom is in love with the bride. Immediately that suggests story development. Who is the friend? Who is the bride? Why does he/she love her? Why is she marrying someone else? Does the friend know the groom? What is their relationship with the groom? What is he/she going to do at the wedding?
You can recognise a good idea because it suggests lots of questions that you need to answer. And in answering the questions, you write a novel.