could use some more information such as year, mileage etc.
what do you mean by "not starting". is there any response to turning the key? if not, you may have a dead battery even though the radio works there may not be enough current to turn the starter. have you tried jump starting the vehicle? if not, try it. if you did and the starter won't turn then it is usually a statter issue.
be sure to check the condition of the cables and that they are tight.
you may have gotten a low quality rebuilt starter that has failed.
does the starter make any noise? Does it 'click" when you turn the key? if so, its probably a battery problem and not a starter problem.
does the engine turn over at all? if it turns but will not fire it is not a starter issue but may be a fuel or ignition problem.
its hard to narrow it down without any information other than what you provided. the usual suspects are a low battery or a poor quality rebuilt starter.
hope that helps
