OK, let's take this one step at a time:
1. How do people measure blood pressure? Blood pressure is measured using a sphygmomanometer. That's a big word that just means a cuff is wrapped around the upper arm and the pressure in the cuff is recorded. See
http://en.wikipedia.org/wiki/Blood_pressure for more information. You're interested in the non-invasive methods ONLY.
NOTE: Blood pressure monitors can be very unreliable if not used in the proper way. I would highly reccomend asking someone like a doctor, nurse, etc. to help you figure out the best way to measure blood pressure.
2. Procedures. You need to have two groups: a control group and an experimental group. The control group will do everything the experimental group does EXCEPT actually play the video game. These things that are the same are your controls. I would reccomend measuring the blood pressure of everyone in both your groups before the treatment and then after the treatment. You can then compare the pre- and post- measurements.
3. Variables. The independent variable is the thing that you change. In your case, this would be playing video games. The dependent (sometimes called the response variable) is what changes because of the independent variable. So, since you are expecting blood pressure to change, your dependent variable would be blood pressure.
Let me know if you need anything else,