Super Saiyan X is a form that is a counterpart of the infamous Super Saiyan. The form is only achieved if you can reach a form that is not linked to the Super Saiyan. The only way to achieve it is by using the False Super Saiyan (which is not directly linked to the Super Saiyan form but only 50% of it is. The other half is the part is your own hidden power. Such as the Kaioken or Mystic form.) The only similar features of Super Saiyan X of Super Saiyan is the blue eyes and the usual hair when they transform.
To Ascend to the Next Form (Super Saiyan X2)
Going to SSJX2 is almost like going to SSJ2. But you need evil (or deep dark rage) to ascend. All forms (besides SSJX) requires it. Goku uses the dark hatred he had as a baby to fuel the form. The dark hatred of the SSJ2 form fuels Gohan. Gotenks can't reach the form, but he can reach level X1.