it would help if you gave some details, like what happens when you turn the key... does it do anything?
If it clicks, start by looking at the starter solenoid, the starter or battery... If it is completely dead or does nothing, then look for a bad ignition switch, a bad fuse or fuse link, a wiring problem, or again a bad starter or solenoid.
If it cranks, then start by looking at spark and fuel...
