I have 2 kids, 3 and 6 on Wednesday. Its funny how different they are, but also how amazing.
As for discipline, I have made my mistakes. Too easy, too hard, too loud!! The yelling seems to lead to them yelling at us later on.
What I found is needed more than anything else is consistency. The first couple of times you try something they don't respond. Whether that be explaining or a timeout. I have found that both together is what works. If they are calm at the time I explain before and then do the timeout. If they are not it is into the room or on the "timeout stair" (when to tired to head up) and then as soon as done I explain what they did wrong and that this was the consequence. If they get off the stair, the clock starts again. It was awful the first few times, but now they know and it seems to work great.
Stores, etc, I take them out as soon as they misbehave. If it is because they want to go home, I take them outside, we stand out there until they calm down and then go back in. That one was tough at first, but I had to be otherwise they would know they could control our lives.