Your faculty mentor will play an important role in your senior project, so it's important to choose wisely. Decide the general area in which you'd like to work, and then choose a mentor who is knowledgeable in that area and will be able to motivate you to succeed. Feel free to talk with a number of faculty members about potential projects and appropriate mentors.
Some projects from the past involved mentors outside of the Department of Computer Science. This is acceptable, but you'll need one official mentor from the department as well. This mentor will arrange your registration with the registrar and serve as your administrative liaison with the department.
You should plan on meeting with your mentor regularly throughout your senior year. Generally speaking, meeting twice per month is common, with more or less frequency as the situation requires.