I read that when your plant get big enough you should cut off the top of it so i grows nice and big. How old should a plant be before you want to do that?
Trimming. I prefer training to trimming though, as I've heard the energy it takes to regrow that, takes away from the energy it could be putting towards flowering and other growths. Tying = tying down certain parts of the plant to made it more wide than tall, allows the leaves to get more sun, and makes the stems stronger. Many of our experiances growers here do that. (sid anyway)
To top or not to top that is the question. Everyone has a different opinion on this. You will get to know which technique (top/tie/FIM/SOG/SCROG) works best in your conditions after a short while. 420shade has the right idea. Test a variety of methods. See which you find easiest/most fun/most productive/best quality and use that. There is no universal right way to grow to grow a cannabis plant.
i have toped befor and i feel it is stupid and training is far better, because new vertical growth changes quicker and teh plamnt size is much bushier.