We need a little more info. Is the engine the 1.6L or 2.0L? Is the transmission manual or automatic? Is there an alarm system on the vehicle, either factory or aftermarket?
The starter is on the back side of the engine, under the intake manifold, so it's a little difficult to get to. Sometimes you can just give the starter a couple of times with a hammer and it will start; if this happens, it's usually the solenoid that is sticking and replacing the starter motor assembly would be in order. There is a black/white wire that attaches to the starter solenoid on the starter motor; 12-volts of power should be there when trying to start the engine. If power is getting there and the starter doesn't turn the engine over, either the starter is bad or (less likely) the engine is locked up. If power is not getting to the solenoid when trying to start, then it could be an open or shorted circuit, a bad ignition switch, bad neutral safety switch, a bad relay... If we knew more about the vehicle, I could go further with that. Also, make sure the "START" fuse is good. In the meantime, Autozone.com has free repair info, including the starter wiring diagrams, at their site if you register an email. Also, NICO Club allows free access to their Nissan factory service manuals at their web site. The "SC" section of the 98 Sentra FSM would have wiring diagram, circuit description and diagnostic procedures that might be helpful to you.