Digression - Basic Tectonic Plate Geology
The movement of Earth's tectonic plates has an enormous effect on its terrain. Most important to this are the boundaries of the plates, and there are two factors that come into play: the type of crust at the boundaries and the type of boundary.
There are two types of crust: oceanic and continental. Oceanic crust is exactly that: crust that forms ocean floors. Continental crust is that which creates landmasses, although not all continental crust is above water.
There are 3 types of plate boundaries:
Transform boundaries, where two plates slide past each other (rub the palms of your hands together for a visual aid). California's San Adreas Fault is an example of a transform boundary. Transform boundaries produce many earthquakes.
Divergent boundaries, where two plates are moving away from each other, creating oceanic ridges and continental rifts. The Atlantic Ocean was produced by divergent plate boundaries between the North American + South American plates and the Eurasian + African plates. They were once joined together in the Pangea supercontinent, but the plates movement spread them apart and the divergent boundary created an ocean. Africa's Great Rift Valley is also being caused by a divergent boundary.
Convergent boundaries, where two plates are colliding. This creates mountains and/or oceanic trenches or island chains, depending on the types of crust that are colliding. Convergent boundaries create a process known as "subduction", where the crust of one plate slides underneath the crust of the other. This creates volcanic activity along the line of subduction. This will be important later.
Transform boundaries have a lesser impact on terrain than divergent or convergent boundaries, although it is important to remember that they do cause earthquakes!
Divergent boundaries occur between two continental crusts or two oceanic When two oceanic crusts meet at a divergent boundary, it creates an oceanic ridge, not to mention new crust. These are basically crust formation points, and you should have one or two on your map. Example: The Mid-Atlantic Ridge.
When two continental crusts diverge, you get a rift valley, and over time, they separate into two landmasses. Example: Africa's Great Rift Valley.
Convergent boundaries between two oceanic crusts eventually create chains of islands (usually in an arc). The subduction causes volcanic activity that eventually builds up into islands. Examples: Aleutian islands, Phillipines.
Convergent boundaries between oceanic crust and continental crust creates volcanic mountain ranges on the edge of the continental crust, because of subduction. Examples: Andes Mountains in South America, Cascade and Sierra Nevada mountains in the USA.
Convergent boundaries between continental crusts creates dramatic mountain ranges as the landmasses smash into each other and buckle the landscape. One crust can subduct beneath the other. The classic example is The Himalayas. This was caused when the Indian Plate crashed into the Eurasian Plate at a high rate of speed (still very slow, however). The Indian Plate is slowly subducting beneath the Eurasian and is creating the tallest mountains in the world, as well as the Tibetan Plateau (the highest in the world).
For a visual of most of the scenarios:
Also, see the image of the Earth's plates.
So now that we know how the different boundaries and crusts effect the landscape, we can take the next step in producing our map.