I'm going to get my first arduino but can't figure out what's the latest one and what to get.
At first I'm just going to learn how it all works.
The "latest one" is technically the Due (IIRC) - but you don't want that one. You want the one with the most support, and the quickest for newbies (like yourself) to get acquainted with. Later you can purchase a more powerful Arduino with more capabilities as your needs increase, and as your experience grows.
For those reasons, you should start out with a standard Arduino Uno, from a major supplier (don't be tempted to go cheap with a chinese clone/knockoff - there are differences that can trip up a newbie). If you are in the US, then something from Adafruit or Sparkfun would be where to start; if you are elsewhere in the world, there are other large suppliers that can help.
My first real project would need to drive two stepper motors. They have to be able to spin perfectly in sync in one setting and then spin perfectly opposite in another setting.
On the spinning, we need to input how may spins in total it would make.
To visualize how it would work, think of spinning a rope in one setting and then twisting a rope in the other.
I'm thinking a tft touch screen for the selection of settings.
So any advice on which arduino unit to get would be great.
Most of this can be done with a standard Uno - sans the touchscreen (but even there, with the proper device - maybe something communicated with via SPI or I2C).
But first - you need to get familiar with the hardware and software aspects. So, get the Arduino, get a starter kit of parts (or build up your own - or use what you have if you already play with electronics and/or other microcontrollers) - and dive in with the examples, and tutorials online.
Once you are really comfortable with the system - then work on getting your steppers to move, and expanding out to the touch screen and other peripherals.
Hope this helps, and good luck!