The simplest way to choose a motor combo is to look at all the FT designs, find which one is closest to the plane you're building, and then use the motor and prop recommended for that design. If you want to get more in-depth than that, we can help.

Kv is a mostly theoretical number. It tells you how fast the motor should spin for every per volt applied when it isn't under load. So, if you put 1volt into a 1000kv motor with no prop on, it should turn approximately 1000 RPM. If you apply 10v, it should spin 10,000 RPM.

Your battery determines the voltage applied. A 2S battery has two cells wired in series. A 3S has 3 cells wired in series. Each cell puts out an average of 3.7v (4.2 when fully charged, closer to 3.4 when fully discharged). When you wire batteries in series, you add the voltage up, so a 2S battery puts out around 2x3.7= 7.4V and a 3S puts out 11.1v.

When sizing a prop, the bigger the prop you put on or the higher the pitch, the more amps it will draw at full throttle. It takes more power to turn a 10x4.5 than it takes to turn a 9x4.5. Similarly, it takes more power to turn a 9x6 than to turn a 9x4.5. If the prop you put on is too big for the motor it will draw too much amperage and cook the motor. If you choose one that's too small you won't be getting all the power that motor could potentially give you.

Volts x Amps= Watts, which is a measure of power. A rule of thumb for sizing your power system is 100 watts of power per pound of airplane. Then you have to figure out what motor and prop combination can effectively deliver that amount of power.