You can only serve on a jury if you are a citizen. If not, then on the form that you were sent, there is a check box which should say something to the effect of "I cannot serve" and on the portion where you have to explain why, you can write that you are not a citizen. Even if you are a citizen, you can excuse yourself from jury service if you can show that serving on the jury would be a great financial burden on you. Whether you are excused or not depends on the judge.