How to make a strong mug of instant Coffee? I am not an avid coffee drinker.  

Yesterday, I tried Bru Gold coffee brand. I mixed one teaspoon milk with 1 teaspoon coffee and added the remaining 200ml hot milk premixed with 2 teaspoons of sugar.

The result wasn't great. The coffee was NOT strong nor did it contain any froth.

I want strong, not bitter coffee.
How do I know how much coffee to add to how much milk?
Secondly, do I have to mix coffee in whole milk and then boil the whole thing like it is done for tea?
Does the amount of time I spend in mixing coffee with milk also have an effect on the outcome?
What is too bitter is likely to be primarily opinion-based. That said, when I make instant coffee, I usually use one teaspoon per 200-250 ml of water - not milk. The result is a nice, even cup of coffee, which is not what I would characterize as strong. If I want strong coffee, I will use as much as four teaspoons to the same amount of water.

The milk will have a mellowing effect on the end result. In combination with the sugar, it has been my experience that the milk masks the bitter taste of coffee that many people do not enjoy.

I would recommend playing around with how much coffee you use. Once you get the strength you want, you can add or remove sugar to reduce the bitterness.