Unit Root


I am required to test for a unit root using EViews statistical package. To do so I am using an Augmented Dickey-Fuller test. The package; however, requires that I specify whether the test is:

1) In level, first difference, or second difference

2) contains a constant, a time trend, or both (I do not have a time trend in my estimated equation).

3) The number of lags.

I basically don't understand the theory well enough to know where to begin when making this selection. If someone could give me a hand that would be greatly appreciated.