Basically, don't freak out too much. If it's stuff you've made yourself you know how old it is, and as long as it's washed and kept cold and covered it's definitely fine within 24 hours. Just watch dressing (make sure it doesn't contain raw egg), and probably add your tuna/cheese and stuff close to when you are going to eat it.
As long as you're avoiding the 'danger foods' (shellfish, cold rice, deli meats, salad bars, and anything sitting within a dodgy temperature for any length of time) you'll be fine.