I wanted to find the inicial point of a stepper motor, if it exists, so I could rotate it always in 90 degrees or 512 steps (2048 steps for a full rotation). I've put four cups in the stepper motor and I want to use the degree 0 for cup 1, degree 90 for cup 2 and so on. I'm using it with Beaglebone Black with python language. So far I've only get to move the motor giving him a number of steps. I'm using the Adafruit_BBIO library to control GPIOs from Beaglebone.
Is it possible to get motor's initial position or move it to a inicial position? I've never used a stepper motor before.
Thank you.
No - it is not possible to determine the exact position of a stepper motor without additional information (inputs). As you've noticed, you can only move a certain number of steps, but unless you know where you started, you won't know where you end up.
This is usually solved by using another input, typically a limit switch, at a known location such that the switch closes when the moving part is directly over that location. When you first start up, you rotate the stepper until the switch closes, at which point you know the current location. Once you have calibrated your initial position, THEN you can determine your exact position by counting steps (assuming your motor doesn't ever slip!)
You see this a lot with inkjet printers; when you first turn them on, the print head will slide all the way to one side (where there is almost certainly some sort of detector). That is the printer finding its zero point.
Some alternatives to a switch: 
If you don't need full rotation, you can use a servo motor instead. These DO have internal position sensing.
Another hack solution using a stepper would be to place a mechanical block at one extremity that will prevent your mechanism from passing. Then just rotate the stepper one full revolution in a given direction. You know that at some point you will have hit the block and have stopped.  This isn't great; you have to be careful that running into the stop won't damage anything or knock anything out of alignment. Due to the nature of steppers, your step count may also be off by up to 3 steps, so this won't be super high precision.
Stepper motor 28BYJ-48: How to find the angle 0? Or its initial point?