I would recommend 6-12 months (12 are better) in a hospital unit - med/surg, stepdown or ICU - before specializing in dialysis. Although some outpatient dialysis providers do hire new grads, as do some inpatient units, you would be at a disadvantage dealing with this medically challenging (due to comorbidities, as pp described so well) pt population.
If you're interested in outpt/clinic dialysis, many companies do have an orientation program (6-8 wks in general), which often includes classroom as well as on the job training. However, they will only teach you how to do dialysis, but as an RN you will be expected to be able to handle anything that can happen in a unit (e.g., pts with symptoms not directly related to dialysis, such as MI, stroke, GI bleed; power outages; cardiac arrest). Also, you will have to supervise UAPs and sometimes LPNs; in dialysis, this can be challenging since the tasks they do are similar to your own tasks. What exactly these are varies from state to state (e.g., unlicensed personnel may or may not be able to push IV heparin - are you horrified?!), but it adds to the challenge of this environment. You will also need excellent assessment skills and knowledge of meds, both best learned on a general hospital unit.
If you do want to start out in dialysis, I would recommend a hospital inpatient unit. Usually, they have only RNs and may have an excellent orientation program. In the hospital, you will definitely learn more (charting, meds, some procedures in addition to dialysis), however keep in mind this is still a specialty area.
One final consideration: As I said before (sorry for repeating), dialysis is very specialized. It may limit the areas of nursing you may easily transfer to down the road. Although not impossible, it's harder for "seasoned nurses (what many consider nurses out of school for more than a year to be) to find a good basic orientation or internship program. But then again, like many of us, you may never want to leave this area!
Best of luck to you,