Split the sheathing on the cable leaving about 1/4in still covering the wires. Cut the sheathing to remove it and strip off about 3/4 inch of the plastic insulation from the end of each wire. Avoid nicking or scratching the copper wire while removing the insulation.
A wall receptacles with two outlets. These are available rated for 15 and 20 amps, as well as special ground fault circuit interrupters (gfci) and isolated ground receptacles.
If the existing receptacle or switch is connected through the holes in the back, push a jewelers screwdriver or similarly shaped tool into the slot next to each hole to release the clamp pressure and pull the wire free.
When running new wiring pay attention to the amp rating for the circuit you are extending. For a 15 amp circuit use 14 awg (american wire gauge) cable, for 20 amps use 12 awg, 30 amp: 10 awg, 40 amps: 8 awg, 50 to 60 amps: 6 awg. Most lighting and receptacle circuits will be 15 amps and make use of 14/2 or 14/3, but some of these circuits may also be 20 amps, so be sure of the rating and use the proper cable and devices for your project. Depending on the gauge and box depth, there is a limit to the number of wires allowed inside an outlet box. If adding new ones would force exceeding the limit it may be possible to add a junction box to the circuit.