I work in Nebraska at a fencing company.My hire on rate of pay is
$13.00 per hour. I have worked their for about 4 months.Well they posted a
bulletin saying that all their employees needed to get a class "A" CDL by
a date,Or they were going to take .50 cents from your hourly wage,Well I
just got paid Friday and their was a letter with it that said they were
taking the .50cents an hour tell I get the CDL.So my question is this> Is that in fact legal,can they really take money from me since I got hired at $13 and it was on paper.<If you could give me any info on this that would really help me out.Thanks

Short of a union contract or bona fide employment contract, yes, your hourly wage can legally be lowered as long as, in your state, you received notification of it before you worked at the reduced rate of pay. I'm unclear of the timing of the notification vs. the effective date of the change from your post. All the wage payment laws require is that you be paid minimum wage.

Can you go ahead and get the required license ASAP/

I don't know what "paper" you're talking about, but it was likely no more than an offer letter or similar, which hardly ever rises to the level of an enforceable contract.