I am a step-dad, my daughters were six and four when we met, and twelvve and ten when we married.
The golden rule is the same as for biological parents - present a united front. All children try and play the 'soft' parent off against the disciplinarian, and as the step-parent, of necessity, this will be you.
You must agree in advance to back each other up in front of the children, even if either of you thinks a discipline decision is harsh - discuss it afterwards when they are out of ear shot. That can apply just as much to your wife - no-one is perfect in this matters.
Consistency is the root of good discipline - once the children know that if you say 'No', it's pointless asking mum, because she is guarenteed to say the same, then you have a consistent message. If you weaken, the children will expolit your wekness, not because they are scheming or evil, but because that is what children do.
Watch how your wife handles things, and follow her lead, but ever ever side with the children against her, or allow her to do it to you.
Follow that simple rule and everything will be fine - my oldrest girls are thirty-one and twenty-nine and we have another seventeeh-year-old daughter as well. It's not always easy, but it is always worth it.
Remember, love and respect don't come as a package, with birth, or step children, give them, and you'll receive them.
Good luck!