If I need to have an impairment that will last at least 12 months, does that mean I need to wait a year before I apply?

No.  If you believe you will be unable to work for a year or more, it is a good idea to apply as soon as you can.  Currently, an average case in Nebraska already takes about 18 months from the date you file your initial application until you get your hearing in front of a judge.  Delaying your application could not only cut into your retroactive benefits, but it will cause you to go longer without income and make it more difficult to afford medical treatment.