In addition to DAWW's correct response:
It is a myth that if one employee is offered health insurance, all employees must be. That is an oversimplification of what ERISA requires. The plan document DAWW refers to may, quite legally, say that certain classes of employees are eligible for or not eligible for health insurance. For example, where I work we have 26 different classes of employees. Of the 26, probably 20 are eligible for health insurance. The other 6 are not, and the plan document specifies this. In addition, even employees in the 20 eligible classes have to meet one of two criteria - either an hours requirement or a wage requirement - to be offered it. The plan document specifies this as well. Employees in the 20 eligible classes who do not meet either of the two criteria are not offered health insurance, and that is legal.
My state has arguably the strictest requirements of any state in the US with regards to employer-offered health insurance, at least for this year. Come 2014, the rules change. But for now, we remain one of only two states that require employers to offer health insurance at all (the other is Hawaii).
IF you are of an eligible class of employee AND you meet all the eligibility criteria as defined by your employer's plan document, then your employer must offer you health insurance. If you are NOT an eligible class of employee OR if you do not meet all the eligibility as defined by your employer's plan document, then your employee not only need not, but may not, offer you health insurance.
As DAWW said, only your employer can tell you if you are an eligible class of employee or if you meet all the eligibility criteria.