How do I add a switch/light and two outlet from a single power source?
Several issues to avoid:


Don't hide any junction box. It can't be under the insulation or inside a wall.
Wires should enter the junction box still in the outer insulation, and then you'll need several inches inside the junction box to make the connections. If the existing line doesn't have a lot of slack, then you won't have the space to do it.
Circuits and the associated wiring have a capacity that you cannot exceed. Doing so will trip the breaker in the best case, or cause a fire if the breaker is upgraded without changing the wiring.
Wires going into the wall need to be stapled to the center of the stud, so you're going to open up a wall eventually. Do not attempt to fish them down the wall and leave them hanging free.


Like ChrisF says, if you don't know what you're doing, you would be wise to call a professional.