You kind of have to be demanding, not necessarily a jerk.

Don't let them get away with stuff. Be consistent, be firm, and have high expectations.

Respect comes with time. Being a kid's "friend" will not win you any respect from anyone - least of all anyone that doesn't like that kid.

You can be warm and still be strict. You can be friendly but still stern. But have an expectation of behavior from the beginning to the end of class, and when that expectation is not met, hold kids accountable to it.

Nothing causes kids to lose respect for a teacher faster than a teacher playing favorites or lying. And while you may not see it as lying, saying you're going to do something and then not following through - even with "You'll pack up and sit out if you talk again" then they know that you won't have any bite to back up your bark.

Finally, don't use ultimatums, and don't punish the entire class for the actions of a few. Have procedures for all students and follow them.