Depends upon the cause. There are some OTC medicines, I believe--mostly effective if the cause is digestive. Seems to me something like "Milk of Magnesia," or the somewhat similar "Kaopectate" can sometimes be effective--though it's been a while, thankfully, since I had to deal with nausea, so you should probably check labels carefully.
But you can also be nauseated by inner ear or balance problems, vertigo and motion sickness, if I'm not mistaken. Dramamine used to be the standard med for the last, and may still be.
Nausea can even arise from emotional stressors such as anxiety, horror or disgust. (The good part about that scenario is that the nausea will almost certainly go away fairly soon on its own if the cause of the emotion ceases, or even just if you adjust to whatever is bothering you.)
So, all in all, I think the standard advice applies: if the problem persists, and especially if you can't tell what is causing it, you should see a doctor.