That's a tricky one, because usually agents want to know they can pitch your first book as a stand-alone. Although they are typically happy if a book has series potential, most publishers will not take a chance on a debut author beyond book #1. (I mean, they will not offer multiple book contracts right away).
That is not always true, of course, but it is very often true. The agents and/or publishers want to know that book one will sell before they start offering contracts for two or three (or more) books. Also, they want to know that the author CAN deliver -- as in, meet deadlines, make necessary revisions, work with editors, etc.
Of course, sometimes a book is so attractive to an agent and/or pub. that they offer a multiple book deal right away. But that is rare.
Any way you can rework your first book to be a stand-alone that can still lead into a series? You don't have to answer all questions or solve all dilemmas in book one, but it should feel like a satisfactory ending (not a true "cliff-hanger.")