Make sure you're teaching by example. Your child is much more apt to learn behaviors when you exhibit these behaviors. It's been proven that children learn by mimicking those around them. So if you're telling your kids not to yell in the house and you're doing it, they're more likely to follow what you do than what you say.
The most important thing to remember while disciplining your children is consistency. Parenting during the terrible-two's can be challenging, for both little one and adult. At this stage children like to push the limits of what is and is not allowed. Make sure that you set clear boundaries and are consistent with your time-outs so children know what is expected of them.
Make sure that your child is getting enough sleep each night. If they do not get enough sleep, they tend to lose focus throughout the day, which could prevent them from doing school work and other activities. Try not to let them eat or drink anything sugary for at least a few hours before bedtime.
If you want to encourage your child to study longer, don't force them to sit at a desk for hours on end. Your child might feel more comfortable on a bean bag, or in the imaginary fort in their room. As long as the location is well lit, it can serve as a great study area. This just might motivate your child to study longer.
Hopefully you've started to breathe now. You can parent and you are going to do a great job at it. We all make mistakes now and then, it's those mistakes that help us learn the tips we've given you. Keep learning and keep loving your child and you will raise a happy healthy child.