The first thing to sort out is how many times a year you are actually getting sick. This is because it can be quite normal for someone to several colds during the winter season, particularly if they are in a job with a lot of exposures (for example, working in a school or hospital). Having multiple colds a year does not necessarily mean that there is something wrong with your immune system or that you are missing some essential vitamin.
Although frequent illnesses can be a sign of an immune problem, these illnesses are usually more severe, certainly more than just the common cold, so I would not necessarily worry
There is no great sure-fire way to prevent getting colds. Although there are many herbal and vitamin preparations that are marketed as helping to prevent colds, in the end they probably have minimal effect. Taking a multivitamin a day will certainly not harm you, but it is not likely to decrease your frequency of colds.
What you can do, of course, is try to avoid exposure to sick people when able and to make sure to practice good hygiene yourself (covering mouth when coughing, washing hands).
Please talk to your primary care doctor
for more information!