How can I support my immune system with MGUS?

Mgus itself is benign condition. There is no treatment for it. Viruses stop gap are vaccines in a limited fashion. But even after you may still get sick. MGUS does not cause you to get sicker or more susceptible to diseases that I know of. With multiple myeloma that is more the case.

I have too much IgA and lack of IgM and IgG that mean my "bar is open" to viruses and my body can't fight towards them. Immunity don't work as well as normal.

@sussu Being in a school environment with a shaky immune system can be difficult, definitely!

Supporting yourself with immunity boosters like making sure you get your vitamins and minerals, enough rest, stress-free as possible, healthy diet. Clean environment as best you can [hands washed, mask on, limited exposure as you can to sickness, etc]. From Harvard University Health is this article: https://www.health.harvard.edu/staying-healthy/how-to-boost-your-immune-system I imagine it will take a lot of work on your part to do this.
Ginger

This is very similar to what I have with SMM large amount of IgA. IgM low (borderline). IgG normal though. So maybe I have the resistance present. Good handwashing. Mask. Foam in and out. Carry cleaning gel? Vitamins or other OTC for immunity boost.