What Frequency
The frequency your FPV transmitter runs on is the most important thing to consider. Nowadays most FPV equipment runs on 5.8Ghz since its legal to use for FPV use in most parts of the world. Depending where you are located you can also use 900Mhz (popular in USA), 1.3Ghz, or 2.4Ghz to transmit your FPV video signal on.
Why is 5.8Ghz the best frequency?
5.8Ghz is by far the most popular frequency used for FPV, but it is not technically the best. Higher frequency signals can carry more bandwidth (which is good) but have a much harder time penetrating obstacles such as walls/ trees (which is bad). So if you use a lower frequency transmitter such as 900Mhz you will most likey have a more reliable signal since it can pass through objects easier, but since 900Mhz is not open for use in the UK, and most of Europe we cant use this for video 
. Also 5.8GHz antennas are much smaller which makes using 5.8Ghz transmitters easier and cheaper. With the correct setup you can easily fly very far with a standard (and fully legal) 5.8Ghz FPV setup, with some pilots reporting 2Km range.
You can also use 2.4Ghz for FPV legally in Europe, however since almost all R/C equipment (and WiFi) runs on 2.4Ghz, you will usually receive lots of interference on this band. Or if you start flying far away your onboard 2.4Ghz FPV transmitter signal could drown out your 2.4Ghz R/C control signal which is sent from the ground, resulting in your drone loosing control 
.
So you dont really have much choice when it comes to frequency selection for video, which is why most of us use 5.8GHz.