Nice thing about "knowing nothing about electricity" is that you have an open book to start filling in. Nobody starts out knowing everything, some have a little basic understanding while others might be employed in something that helped them build their knowledge level. You have the opportunity to perform some online research through various free internet sites that specialize in electrical info to expand your understanding and ability.
The small 2s battery will work..for a while...,but you should look at the video TX rate of consumption to determine how long that battery will be able to maintain enough voltage to power the TX. The TX data sheet should have an average consumption rate printed on it somewhere, while the manufacturer website should also have that info somewhere for your review. Never buy anything electrical that consumes power that does not have that info openly listed somewhere. Same thing with video antennas that don't have a dB gain rating listed. The video TX might consume a couple hundred or more mA/hour and any battery selection will limit the amount of time it can be used as the power supply between charges. The 2s is at the bottom side of your voltage requirements so the use time will be limited by the amount of time it takes to draw the battery down to 7.4v with a video load on it. Nice thing about using a separate battery for vid TX power is that if the video battery fails everything else stays working.
Learning does not have to be hard work if you are learning for something you plan to have fun with. It may not always be easy but with a little effort and progressive learning it all comes together.