This number one rule of spells works with the 'like attracts like' principal. So that when you spell cast - you must believe, know, feel, see that what you want to have happen will.
This is where is can be problematic for people. As we are so full of the emotion - it might be worry over money, or heart-ache over lost love. And that feeling is incredibly prevalent, and it requires a huge commitment, conscious decision and hard mental and emotional discipline to choose to focus on what it is you want as if you have it now.
So whilst you cast the spell - and a spell in, of, itself is indeed powerful - you need to be focused on what you DO want. And then even after casting that spell, to support it happening - it requires you to move into the emotional state of certainty that what you want to have happen will. Many would call this faith - and faith is incredibly powerful!
"Reality is wrong. Dreams are for the real."
The main thing you need when spell casting is belief, in both yourself and the spell. If you don't believe strongly that the spell will work or you don't believe in yourself that you can cast the spell then it may not work. Also make sure you're not being distracted, if something else is in your mind you may not be putting all you have into the spell, try and focus on exactly what you're doing first and then try and cast the spell again. The last thing has to be PRACTISE, I'm going to bore you with t e old quote but it is true.
Learn your Basics first before spells. As stated above, belief and will is the key but concentration is vital as well. It's best to meditate, ground and center before a spell(or a ritual) and then ground after casting the spell or doing rituals.