You first have to raise your energy, than and only than you can successfully cast spells. There are several techniques for raising energy.
2. Getting rid of your addictions. You can recognize addictions by a short increase of energy, followed by a drop in energy afterwards. Addictions can be everything: Alcohol, sexual, food, complaining, mobile phone, some people, etc. The most import thing is to be aware of your energy levels. Which activities lead to an increase and which to a decrease in your energy level. As said: be aware of addiction. Because they temporally increase your energy level. But in the long run they stop you from spiritual growth.
3. Grounding. Letting the energy of the earth flow through you.
4. When you energy level is high enough you can start with casting spells, mantras or healing. If you're energy level is not high enough it simply will not work. Casting spells is a form of focusing and concentrating energy. You can start with a simple spell: I am the most powerful being in the world. Stop after every time you say this sentence and open your mind (clear it from thoughts and visualize that you're opening a door to the universe). Let the energy flow in. Repeat this as often as you want.
Personally, there is no ''one way'' to energy, and it can come in a variety of forms. From spirits, meditation, leylines, psychic vampirism, etc.
Find one that appeals to you, and do it. Regularly.
See how you can exercise and get more power as well. Magical power is simply how far you can push in one direction, or another, or another...
Entities with more energy than power are ''fat'', whereas those with more power than energy, are ''sparks''. You want to be balanced, though with slightly more power than energy.
Energy comes naturally, though, but is wasted too much, on things like self-importance, addiction, etc. so to be rid of those things, I would advise learning how to eliminate them. Having enough energy saved is called, in Castaneda terms, impeccability, and you can read about it in those books.