Immune systems rarely need a boost, they just need the tools to function a peak performance. The number one way to kneecap your immune system is to not get sufficient sleep. Flu vaccines have been shown to be dramatically less effective when sleep deprived individuals took them. Your body needs 7-8 hours, full stop.
Supplements are a racket. Most are concentrated doses of micro-nutrients that your system immediately flushes because they're not in a form your body can absorb effectively. You get these nutrients best through whole foods. Want vitamin c? Eat an orange. Increase the amount and variety of fruits and veggies in your diet.
Two side comments: Rain/getting wet does not cause colds. Neither does eating/drinking cold foods, another misconception I have heard. However, cold, dry air (like from an AC) does dry out our sinuses which makes us more vulnerable to cold viruses. Getting outside and drinking warm drinks (like the teas deardoveswings suggests with helpful herbs) will keep our mucus membranes hydrated and effective barriers against contagions.
Finally, COVID is still around and we are in the latest wave. Avoid crowed indoor venues when you can. Wear a mask when you can't. Especially if traveling on planes.