Which arduino should I start on to learn about it?
I would simply say: buy a startkit like this one.At the end it is not important which Arduino board you'll start. They are (almost) the same*. At the very beginning you can start with Uno or Nano and even Leonardo could be very good. Those boards are very similar and you can program them in the same. Even the pins location and designation is not really different.
The most important thing about Arduino is to understand how to work and program and interface such a board with the sensors. In this sense I would choose a Uno board (or even 2009) which is less expensive and very easy to find overall. I don't really know how much experience you have, but it is likely, at the beginning, that you invert polarities of batteries, you drain too much power, or you invert some pins and damage the microcontroller. In that case doesn't make sense to buy the last powerful mega expensive board on the mark. If you get a Uno you can do almost everything and have for the next month a lot to learn. 

To change from Uno or Nano to the other board it is not very difficult. Once you understand the basis you can switch to every board you want. If you need more power and memory you can buy a Mega. If you need much more power you can buy a Due/Tre/etc...you need just an upgrade not everything from scratch to learn.

If you are really new to Arduino the best way is to buy such a learn package as I showed you above. That have everything you need to understand basic of programming till to RC-Servo movement and motor drivers.

Happy new year.

*: stay away for the moment from the Arduino Due/Tre/Zero/Yuan/.. series. they are too expansive and require a lot of attention (3.3V, max drain current from the pins, pull up, pull down...etc..). Here is the risk to get your board damaged within few time very high. And you'll spend a lot of time thinking that the board has some problems.