It depends on the person. Some authors are inspired by their own experiences, others by things they have been told, others by stories they read in newspapers, others by everything they read…
What you do need to bear in mind is that an idea is not the same as a story. A story is like a puzzle formed by many different ideas. You may have an idea at the beginning, for a character, for a place… but that is just the first piece in the puzzle. So it’s not just a matter of inspiration. It is true that there are some authors who start with one initial idea and then improvise, but I don’t recommend that if you want to create a solid plot and rounded characters. Don’t just wait for ideas to come to you. You can gather many pieces of the puzzle, but they won’t fit together by themselves. You need to think and find the best way to fit them together, and nobody can do that for you, not even inspiration. It doesn’t matter if you have to discard good ideas because they don’t fit into the plot. It’s better to keep these pieces, as good as they may seem, for future projects, rather than put them into the story even if they don’t fit in well.
If you write fantasy, many of your ideas will come from your imagination. To keep it in good shape, it is especially important to read a lot and think a lot too. Don’t worry if your stories seem similar to others you have read. That’s what usually happens in the beginning. If you write a book in which you can tell that you admire a certain author a great deal in every paragraph, it’s not very likely that it will get published, but don’t worry: that is part of your training. You start by imitating other people, almost always unconsciously, until, as you become more widely read, have experiences and reflect on them, you end up finding your own style. A writer is not original because he tells stories that nobody else has told. He is original because he tells them from a different point of view. You will be a writer with your own personality when you have you own way of seeing the world. That will be reflected in your stories, so, however clichéd the plot of one of your novels may seem, it is the way you tell it, your personal vision of these topics, your way of reflecting on them, or bringing them to life, that will make the work original.
If you want to write, but you don’t have any ideas, write about things you know about or which have happened to you. Write a diary, a blog, or whatever, it doesn’t matter if nobody reads it, it doesn’t matter if you only write it for yourself. Every page you write will be another page of practice in which you will be trying to find the best way of writing your feelings and your thoughts, and that’s why it won’t be a waste of time. That way, when you do have a good idea and you are able to develop it until it becomes a good story, you will also be able to put it down on paper in a satisfactory manner. So you should write even if you don’t have any ideas. If you don’t, if you wait for that great story, you might make a hash of it simply because you don’t know how to write it.