Series and sequels are popular, particularly in genre fiction: some series are so popular that their publishers commission book-packagers to produce several new titles a year, using ghost-writers who write to other people’s outlines and plots.
Among the many advantages of writing a series of books is that each subsequent book has an established readership before it even reaches the bookshop shelves, and then goes on to build on the success of the previous title. Consequently, many writers who are new to the game plan to write a series of their own.
I would always advise them against it.
By all means, write that first book. But make sure that it is complete as it stands, and don’t even consider starting work on the sequel until you have sold the first book. Because if the first book of the series doesn’t sell, you’ve got very little hope of selling the second.