I am using a solar panel with a regular 12v charge controller but instead of using 12v batteries i am planning to use li-po batteries with a specific charger. I know that the best way to step down the voltage would be a buck converter but i already have a L7805 laying around. Do i need to use a capacitor at the output ? I have seen some circuits that used it and some that did not and both worked fine. If i do need one how do i determine which one suits the circuit? I am a newb to electronics. I see that capacitors have both voltage ratings and capacitance ratings. I figure that the voltage should be above the 12 volts the charge controller outputs as otherwise the capacitor would be blow up, right? Is the voltage of it relevant? How do i determine the right capacitance?
Read the datasheet. It will tell you exactly what you need to know.
TI recommends a 0.22 µF capacitor on the input. A capacitor on the output is optional, but adding 0.1 µF there can help in some applications. For a battery charger, this is probably unnecessary.
Larger capacitors are not necessary, and may damage the regulator if the input is shorted. (See "Shorting the Regulator Input" on page 13 of the datasheet for details.)
That all being said -- as mentioned in comments, a linear regulator is probably a poor choice for this application, as you will end up wasting roughly 60% of the power you gather (7 V out of 12 V) in the regulator. A buck converter or MPPT will give you much better results.