Hi, Helen. Do you mean to convey that you just moved to a new town and are telling someone in this town this fact? In this case, you'd say 'I'm new in town." Or, 'I'm new to this town.' That's how we'd say it in my part of the US. And no, you would not use the definite article.

i'm new to the town but both will be fine

Yes they both are correct and can work in similar situations