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Prove to yourself and to the world that you can write one decent book. Until then, there is not a lot of point planning a trilogy or a series. Until you can complete one book, you are not going to have any real idea of how bit an idea it takes to create one book, let alone to come up with multiple related big enough ideas. Get a sense of the scale and complexity of one book before you try to plan multiple books. 

And for this reason, don't expect anyone in the industry to be interested in your series or trilogy plan until they are convinced that you can write one book. 

It's not that planning a series of books it hard. Actually, creating a plan is pretty easy. Probably quite a bit of fun. It may or may not be a realistic or achievable plan, but you can't tell whether a plan can be turned into a series of books until you have had the practical experience of writing several good books, or at very least one. 

On the other hand, the market being what it is, the minute an agent or editor decides that they like you first book, the next thing they are going to ask is, "Can you make this a series." The best way to sell a series is to write one good book.