How to improve tesla coil ?

I built a tesla coil, but the output is just a little spark barely visible when i approach a screwdriver to it.
primary is 5 turns
secondary 500t
capacitor is made from 2 layden jars
spark gap is about 3mm
the coil is about 15cm tall
the power supply is a flyback transformer driven by the zvs at 12v
What do I have to do to improve it?

I've been down that road before I'm still building my coil, I started off with flybacks and leyden jars but I found that flybacks didn't have the power for my coil (secondary 3.5inch wide 905turns and 13inches tall) the next step up from a flyback is a ignition coil.
I recently bought a multimeter that could measure capacitance and was able to measure a wine bottle capacitor I had made for my coil months ago (when I was still playing with flybacks) and found it had a ridiculously low capacitance of 0.63nF (the average for a Tesla coil is about 0.2uF). I am now at making glass plate capacitors which will be able to give me 0.2uF easily.
Also if you are using leyden jars I doubt your coil is in tune, I had half inch sparks coming off my coil to a screwdriver, I would recommend, these sites uzzors2k and deep fried neon, find an ignition coil and buy a multimeter that can measure capacitance and inductance. 
This site will come in handy.
If you built your coil with arbitrary primary turns, primary capacitance, and secondary turns it is unlikely that it is tuned at or even near resonance, which is essential for a Tesla Coil.

Formulas for the inductance of different primary and secondary coil types are readily available on the web, so to begin I would use those to determine what you have. You will also need to determine the capacitance of your Leyden jars, for which there are also formulas readily available on the web.

Whatever kind of "top load" you are using (sphere, torroid, etc) will contribute the capacitance of the secondary circuit, and you can use available formulas to estimate what those are.

Once you have these pieces of information, you can calculate the frequencies of your primary and secondary circuits. You can then change them as needed to achieve resonance (or get reasonably close to it) by changing the capacitance and/or inductance on the primary side.

Changing the primary inductance is a common method of tuning, and is usually accomplished by seting up the primary coil so that it has adjustable tap. This effectively sets how many turns of the primary are used.

The top load size and type will also effect turning. If you change your top load terminal, you may need to go back and adust the primary again for best results.

Again, there's tons of good info ont he web, including the necessary formulas to at least approximet the needed values. I'd give them to you here but I don't have them just off the top of my head.