Obiad is the main meal of the day, usually eaten around midday (12PM to 4PM). It is usually translated as lunch (because of the time of the day when it is eaten), sometimes as dinner (since it is the main meal which is often eaten socially – with family members, in a restaurant etc.)
Kolacja is a medium-sized evening meal, usually eaten between 6PM and 9PM. Again, since the conventions for naming a meal of this kind in English vary, it can be translated in two ways: as dinner or supper.
Obiad may be usually translated as lunch in north america, but in Polish it's dinner nonetheless. It shouldn't be wrong if you write obiad since it's the correct way.
What is the difference between "coś" and "czymś" which was mentioned earlier this lesson?
Cases. "coś" is either Nominative or Accusative, "czymś" is Instrumental. So, let's say "Kanapka z czymś" (a sandwich with something).