To be specific, you can select Earth in the 'System Locations Available' list, then Load Colonists will show up in the list next to it, labeled 'Actions Available'. Double click it, or select it and click the Add Move button to add it to the queue. Now click on another location, let's say Mars, and choose Unload Colonists. Your ship will now move colonists from Earth to Mars, then wait for more orders. If you want it to repeat these actions you can click the 'Cycle Moves' checkbox; then each action will be moved to the bottom of the queue once the ship finishes it.
Creating a colony is as simple as dropping people or equipment there. Of course, the solar system has a lot of bodies you can put colonies on, so sometimes finding the right one in this window is quite hard. You can click on the sun icon in the System Map window to open the System View (yea, I know, great names here) which gives you a lot more details about the bodies in the system. This window has a 'Create colony' button which does exactly what it says. If you now check the Task Group window you'll see that this colony is sorted to the top of the list of available locations, making it easier to find.