If it means a specific type of food that someone has never tried before (eg. curried rice salad), then you would use "a". But if you didn't mean a specific food, you don't use the article.
'meal' = „posiłek”; every „posiłek” is a „jedzenie” but not all „jedzenie” is a „posiłek”. ;) For example, if you would pick some berries while hiking through forest, it would be „jedzenie”, but you would have to pick a substantial amount(enough for a 'real' serving) for it to qualify as „posiłek”.