I feel its best to start out with some sort of shield instead of getting into spells. People underestimate the power of your natural ability to defend/shield yourself with visualization exercises. For instance, If I'm approached by someone who has ill thoughts or intentions towards me my guard/shields go up instantly. I have a two step defense: step 1 is a warning and will make all beings, both of this physical plane as well as any other, have a feeling that if they mess with me or try anything that something bad will happen. Step 2.. something bad happens if they mean harm. I have a trigger for when I feel threatened that expands a clear, indistructable spiked ball outwards in all directions around me... I have seen it in action on real people before and to my amazement it worked very well! The guy actually stopped in his tracks and looked as if he was hurt and confused. I felt his presence behind me and turned to see this happen, our eyes locked almost instantly and I think he knew what was up. I doubt he was going to try to physically hurt me (I'm a pretty big guy) but I think he may have had a bad thought about me, tried to make fun of me or something along those lines.
Here's my method, try it out and customize it how you see fit to get the results you are looking for:
Get into a meditative-like state where you start with a clear mind and do the breathing excercises that you have adopted.
Now let your physical self know that this particular session needs to link up your regular senses to your subconscious and the other senses that are for fear and defense.
Use your visualization skills both consciously and subconsciously to see your physical self in the third person and the energies inside of you, the energies that you emit and the energy receivers you have.
Tell yourself that you are now setting up a very strong defensive system that is very efficient at both deterring and attacking threats that you sense both consciously and subconsciously.
Bring yourself back into first person mode gently... feeling your aura and energy that surrounds you. Feel how powerful it is.
Now visualize pulling all of your energy from outside of you as well as from the limits within your body to your solar plexus. Form that energy into a bright golden ball. Feel the warmth and raw power. Spin the ball in different directions so you realize that you have complete control and can make it do whatever you want it to.
Tell yourself that you are now going to set up stage 1, your warning system to those you feel threatened by.
Tell yourself "When I sense a threat on any level, this is what this energy will do"... now expand your energy out past your body in a ball (you can make this energy a puke green or whatever color you want) and put the thought of "I don't think I want to mess with her/him, He/she is a good person but might do some damage if I provoke him/her!" Put the thought that you want the threat to think, feel and say to him/her self in the energy so when it reaches them they absorb the thought/feeling as their own.
Bring your energy back in to a natural state, breath and relax for a moment to restore your energy again.
When your ready, gather all of your energy, both out and in but this time suck the energy from everything around you into your ball too.
Tell yourself "This is stage 2. If I still feel threatened, this is what to do immediately before they can do any harm!" Now quickly release a big ball with an indestructible spiked shell out all around you that only hurts those that wish you harm. When it releases it should be so hard and quick that you imagine it actually effects the physical things around you! Spring it back to normal inside of you quickly, everything going back to normal except that your on high alert for another attack right away.
Now, tell yourself that this session is over but to lock in this defense all the time, even while you sleep or astral project but that it is VERY efficient and won't drain your energy.
Now relax and bring yourself back to physical reality slowly, feeling very powerful and ready for anything if something should come up.
You might feel VERY energized. If so, ground yourself slowly... or you can enjoy your new power for a while :) I strongly suggest to ground and center yourself about an hour before bed or you won't sleep. Do this visualization in the mornings so you can feel energized and confident all day.
This is what I use and it is amazing. I hope this helps and works well for you as well.