Its really going to depend on the type plant/tree you want to root. Some plants are much easier to root than others.
You can, however, buy some rooting hormone your locally owned garden center and talk to an employee about the specific plant you want to root. In our area, during the summer, garden center employees are college students majoring in botany so they are very knowledgeable on all kinds of plants.
You can get a rooting hormone at a gardening center. You just cut off the branch and use a knife to shave off all of the bark at the cut so that all around it is bare. Dip it in the rooting solution and place in soil. Keep it moist at all times. I have better luck with a small branch then a large one.
If the branch is small enough that you can put a glass jar over it, you won't have to worry about watering even. It will work just like a terrarium.
I have not been able to find the rooting solution at Walmart etc. You need to go to a more specialized gardening center.