During my undergraduate years, I worked as a security guard in a variety of contexts. It is usually a relatively-easy job to find, and it fit well around my schedules.
As has been pointed out, the options of working in different locations and different shifts will make it more likely you can find a good "match" for what you do well--but it might work out that this just isn't a job you want to do. Some positions require dealing well with the public in what are often stressful or upsetting times for them. Other positions may mean you work alone and seldom, if ever, encounter anyone other than the people on the shifts before and after yours.
You can kind of sort security jobs into two categories. In one category will be jobs where your ability to deal with the public is a major, if not prime, consideration. This category would include positions at businesses or offices when they are open for business, or at stadiums/concert venues, as well as jobs like "doorman" at a bar or nightclub. You interact with people much more in these jobs, and will often be answering questions or attempting to resolve problems. (I generally tried to avoid these positions, but have to admit that being a doorman/bouncer was one of the more entertaining jobs I've had over the years.)
In the other category, your ability to perform tasks according to a timetable and to make correct decisions in a timely fashion, will be much more important. Examples of these would include factory security (particularly during times when the factory is not working), access security (like at a guard shack as Videodrome described), and warehouse security (often nothing more than walking around and checking that doors are locked, no one has gained unauthorized access, and no fires have broken out).
Jobs like overnight security at museums or libraries often combine aspects of both: you may be alone much of the time, but have to deal with the public at the start or end of your shift.
Around here, most security positions are with contractors: companies who contract with businesses etc to provide security staff. Unless you are "in-house" security (i.e., your employer is the company or institution you are guarding), pay and benefits tend to be fairly low. This can make it easier to get an entry-level position, and gain experience to increase your chances of getting a job as in-house security.
Sometimes it can be very stressful, other times it can be mind-numbingly boring (sometimes in the same shift).
Jim