Whenever you work on electric and dont have a meter to check that the circuit is off,,,doesnt matter if you turned the breaker off,,,always work on it like its live.
They do make little dummy light plugs to check outlets for proper wiring.
Your best bet is to buy a multimeter, can be purchased for like $5 from harbor freight. Their pretty crappy but will work just fine for what you need it for. Google is your friend when it comes to how to use one. Googling anything you dont know about is pretty awesome. Thats how I found this website when I wanted to learn how to grow.
Not talking about you specifically but so many people think that because they know where wires go that they know how to do electrical. Such as just an outlet. If you dont make a hook with your wire in the clockwise direction of the screw its being put on, it can come off, either while your doing it or overtime. There is a whole bunch of stupid little tricks that you learn from doing something for many years.
Shut the breaker off, check outlet for power, if not there take wires off, cut them back but not to short, strip the ends a half inch, make hooks like I stated above. Put white wire (neutral/return, its not a negative) on silver screw, put black wire (hot) on brass screw, put green or bare wire on green screw.
When you cut the wires back, make sure you leave at least 4 inches sticking out of the box, makes it easier to wire the outlet.