talking issues

my precalculus class is having talking issues. they feel like they can talk whenever they feel like it. i try asking them to raise their hands, and then i have trouble enforcing it when some of them start talking anyway. i think i need to talk to individual students, but i also need to work on being more form and less "nice." my problem is that i want to hear what they have to say, but i'm allowing them to say things at inappropriate times, when i'm trying to teach. i need to work on "the look" or being more firm when i tell someone they need to stop talking. i think they get the impression that i don't really mean it because i am too nice or i smile when i shouldn't. i don't like feeling mean, so i try to ask them nicely. but that doesn't work because that's when they think i don't really mean it or that it doesn't actually really matter. i never want to be the teacher who explodes in anger, so i think i go the other direction and end up being too nice. i need to find a middle ground somehow. i need to not let things slide, and actually stop the lesson when they aren't acting how they should be.