Vitamin D - take up to 4000 iu per day (upper recommendation with the NHS). Get one in combo with K2 which helps calcium absorption.
Magnesium
Zinc
Take elderberry syrup when you feel the first tingle of a cold coming on.
Lifestyle
Get plenty of lunchtime sun in the late spring - early autumn without protection - you want to expose as much skin as poss & stay out for 50% of the time it takes you to catch a bit of pinkness (depending on your skin tone).
Food - minimise sugar, alcohol, UPFs, they diminish the immune system. Eat plenty of meat, fish, shellfish, eggs, plain dairy, vegetables, fruit, nuts, seeds, porridge, quinoa etc. Optimise gut microbiome - fresh sauerkraut from the chiller cabinet at the store (or HM), kefir etc.
Moderate exercise in the fresh air. Both too little & too heavy (eg marathon training) are detrimental to the immune system.
Sleep - make sure both quanity and quality are good. Decent mattress, pitch black room, no screens before bed, no stimulating media at bedtime eg reading horror novels! If you have a smart watch/smart phone with sleep monitoring app, look into checking your sleep quality, could be affected by snoring/apnea etc.
Stress - another detriment. Try to reframe your stressors. Try meditation, prayer, practising your faith, even just going outside to a peaceful spot in nature, quitening your mind, inhale & exhale mindfully 🌳