I have written a novella and am now working on the second in the, what I hope to make into, series. The first is about 14 year old Erin who has an odd attraction to older men. She has a close friend whom she met on the internet named Jay. Jay is 25 years old. They have a mutual attraction, but due to Erin being in Indiana and Jay being in Canada, they decide not to pursue a relationship. Erin's made it clear that Jay has been sexual towards her in the past and she has not welcomed it. Erin has a boyfriend of 2 months, Adam, who is 16. Jay knows of Adam, but Adam knows nothing about Jay. Erin soon begins a relationship with Jay but after a month, realizes he is a pervert and backs out of the relationship by breaking up with him on Skype even though he'd warned her not to. Erin doesn't confide in anyone about Jay except for her dog, Leon, whom she has a very close relationship with. Her father abandoned her and her mother when Erin was young, and Erin's mom remarried. After an outing with Adam, Erin comes home to discover Leon in her bedroom, throat slit, and in blood, her wall reads, "I told you not to do it." The police arrive, and Erin is in obvious shock. That is the end of book 1. In book 2, I have ideas as to how it will go, but I'm not sure how to fill in the details.I think Erin will start to become close to one of the younger, attractive police officers and finally confide to him about everything that happened with Jay. They will begin an awkward relationship that is somewhat romantic but won't lead to anything physical. He'll promise Erin that he'll keep her safe. Then, Jay will show up in Erin's room and attack her one night. The policeman will have been staking out nearby and will come to Erin's rescue, putting a bullet in Jay's head. The policeman will realize that he'll have to explain why he was so close to Erin and what exactly their relationship entailed. I'm not sure where to go from there. Any ideas?
Don't write a series, particularly a series of novellas. Write a single, stand alone novel of 80,000-100,000 words. Publishers do not want to see series from new writers, and there is currently virtually no market for novellas. Other than that, it's your story to work out. Trust yourself to come up with your own solutions.
What about a novel that ends in such a way that a sequel is possible? Would that be a better idea, or should new writers stick to a single novel?
Just make sure you know the difference between a sequel being possible, vs. inevitable. The first novel MUST stand on its own, with a strong ending.
Instead of trying to break into the industry with a new series, why don't you try paving the way for a universe for your potential future books to be set in? That way, you already have this grand setting and environment you've forged with blood, sweat, and words ready to use for a good, deep, and visceral story! One large novel (80k-100k words as posted above) that builds a universe for future stories while being a standalone has a better chance being published than one book of a series from a new author.
Correct me if I am wrong, but from what I've read, a novel is considered anything above 60K words. However most print publishers prefer an 80K minimum from first time authors.