It should be fine to cook straight from frozen, but just in case you struggle slightly, i've made you a quick guide:
Note: the assumption is made that you are able to use your arms and hands correctly in order manipulate various objects
Open the freezer door, by grasping the handle firmly in 1 hand, and pull towards you. Remove the pizza from the freezer (you may need to open a second door, or slide out a drawer in order to gain access to the pizza).
Check on the packaging of the pizza for cooking instructions, e.g time and temperature, if the instructions are for fresh pizza, the temperature should stay the same, however you should add approximately 10 minutes to the time.
Turn on the oven, this can be done in several ways, depending on the make and model of your oven, however, usually by rotating a knob on the device itself. The temperature required should be specified on the packaging of the pizza, if it is not, assume 200-220c or gas mark 6-7.
Note: If you have an electric oven, you may need to switch on the isolator switch, which should be located near the unit, although not actually on it. (In order to switch on the isolator switch, simply exert pressure until the switch moves to the "on" position.)
Note: If you have a gas oven, you may need to use the ignition button to light the gas after switching it on, failure to do so can result in an explosion, causing severe inconvenience.
Leave the oven to warm up, approximately 10 minutes should be sufficient.
Once the oven is warmed, remove the pizza from the packaging, this can be done by using a knife to open the box, or to remove the plastic film. (Be careful when using a knife, as it has sharp edges which can cut, if you don't feel confident, ask an adult, or at least someone with a clue to help you).
Place the pizza inside the oven (you will need to open the oven door first, in order to gain access to the interior) on an oven shelf in the middle of the oven.
Note: The inside of the oven will now be hot, don't touch anything inside without protective handwear, again, if you lack confidence, ask someone to help you.
Leave the pizza in the oven for the required time, you can measure this using a time-keeping device such as a watch, wall clock, computer, or mobile phone. If you are liable to forget the time, you may wish to consider setting yourself an alarm as a reminder. It may also be a good idea to check on the pizza occasionally, to ensure it is not burning.
Once the pizza has been in the oven for the alloted time, ensure it is cooked, this can be done by several observations:
is the pizza hot?
is the cheese on top melted?
is the cheese on top slightly browned?
If all of these are true, then it is likely that the pizza is ready to eat. Remove the pizza carefully from the oven, and place on a plate, or cutting board, remembering that the inside of the oven is hot, and the pizza will be also.
Leave the pizza to stand for a minute or 2 before cutting, then, ideally using a pizza cutter, or if one is not available, a sharp knife, cut the pizza into segments, 8 is the usual number; this can be done by repeatedly cutting the pizza in half.
Once the pizza has been cut into segments, you can begin the process of eating it. Pick up one of the segments, using your hand, raise it to your face, while opening your mouth. Place part of the pizza within your mouth, and close your teeth, exerting force to use your teeth to slice through the pizza. Chew the mouthful several times, then swallow. Repeat this process until a) the pizza is finished, or b) you are full.
If any of this is unclear, please let me know, and i'll try to explain in more detail.