When the recipe calls for one cup, basically you need to follow it by using a measuring cup and add a cup of whatever ingredient is needed. Although this is not always the case because you have the luxury of adjusting the ingredients when you are cooking. However, if you are going to bake something and the recipe specifically states that you need one cup of sugar, milk, baking soda, etc., you would have to strictly follow the recipe.
This is one of the main factors why some people fail in baking because they tend to experiment with the measurements. Measuring cups are not that expensive – the plastic cups cost around 5 US dollars while the stainless cups cost around 25 US dollars – and it will pay off if you invest on it.