On the very first day, tell your students the rules of the classroom. Let them know what you expect from them. Give examples of what it will look like when they are following the rules. Likewise, tell them what it will look like when they aren't following the rules. Once you've gone over the rules and your expectations, tell them what will happen when they follow those rules and what the consequences will be when they don't.
One way to reward students for good behavior is to keep a marble jar. Each time you notice that they are following directions and meeting your expectations, drop a marble in a jar while letting the class know that you have done so. When the jar is full, give the class a reward: a pizza party, extra recess, extra free time or a movie.
In addition to whole group rewards, give students individual rewards as well. Each time you catch a student being good, give him a "classroom buck," which is money that can only be spent in the classroom. You can make these yourself or use play money. At the end of each week, open up the classroom store that you have stocked with inexpensive items from a dollar or thrift store, or coupons such as "get out of homework" or "five minutes free time." The students can then use the "classroom bucks" they've accrued to buy these things.
In addition to positive consequences, you must also have negative consequences. When a student does not follow the classroom rules, give the student a "strike" by saying, "That's one." If the student stops the behavior, she stays at one for the next half hour. If she repeats the behavior or does something else inappropriate, count the student to two strikes. And if the student still does not correct herself, say, "That's three," and give the student a consequence. This could be a timeout, loss of recess, extra work or something similar.