LINE is the cable from the panel; LOAD feeds the outlets after this one. Black (hot) goes to brass, white (neutral) to silver, bare to green. Put LINE on the LINE terminals: a modern GFCI locks out if LINE and LOAD are swapped. Use the LOAD terminals only to protect outlets downstream; spliced through with the line instead, they still work but aren't protected. On a real job: breaker off, then prove it dead with a tester before you touch a wire.

    LINE cable from the panel (up), LOAD cable to the next outlet (right)Brass = hot, silver = neutral, green = groundYellow tester: correct = two amber lightsGreen ring: it can go there now (red: not yet)A 15 A or 20 A GFCI receptacle on a 120 V branch circuit, in a single-gang box. It trips when the current going out on the hot and coming back on the neutral differ by about 5 milliamps. The three-light tester patterns are the usual ones: two ambers correct, middle only open ground, red and middle hot/neutral reversed. Follow the device's instructions and your local code, or hire a licensed electrician.