Are you asking how to draw energy or how to focus it? When casting a spell, if you are one that practices the motions and speaking parts to a spell, most of your energy is going to be drawn from yourself and your own intent. That is what makes a spell powerful. It really isn't a matter of the amount of energy but the concentration you have. Depending on your practice you can draw energy from things, some people are able to draw in other people's energy which can drain them, however; you will then take on any negative energy they may have been festering. This is why grounding is important.
I practice working with nature and drawing energy from nature. It is why I prefer to work outside in my woods or use plants. There is no right or wrong way, in fact some people don't even cast physical spells but rather they meditate. You will learn which one works for you but it always comes back to your intent.
When you cast, to direct energy isn't necessary. In the movies they show individuals shooting spells out of their hands, which this is very cool and charming for the movies, that is not exactly how energy works. Your focus and intent on what you want to happen is really how you direct your energy. I am not disregarding one's ability to use their hands for energy work. I often will place my hands on trees in my woods and can feel the pulse/vibration of their energy. Some practice Reiki healing, in which you would focus your energy into your hands. If you get good enough you can feel the vibration or the pulse surrounding your hands as you concentrate healing.
Hopefully this helps a little! Good luck with your casting!