How can I prevent an infection?

"I think I'll have mononucleosis in a few days/weeks, because I've almost for sure got infected" -- The incubation period for mono is 4-7 weeks, not days.

"One of the "symptoms" of mono, is amigdalitis, which is an infection (I think so, at least xD)." -- Where did you come up with that?? Amigdalitis is Spanish for tonsillitis and tonsillitis is also a symptom of a bad cold, the flu, Strep throat and other URIs. Tonsillitis means inflamed or infected tonsils but the word "tonsillitis" doesn't identify the reason behind the inflammation/infection.

"So, what should I do to prevent this amigdalitis from being too serious??" -- I don't think tonsillitis is the real issue. If you develop symptoms of mono (and be sure to Google those so you know what they are) 4-7 weeks after you think you were exposed, see your doctor for screening.

"Apart from vitamin C, which is probably the most important." -- No, it is not the most important. C is great for tissue repair but can't do anything to fight a viral infection, which is what infectious mononucleosis is. However, take that and zinc together as well as all the rest of your vitamins and minerals.