I am melting some chocolate on pan so I read that melting with water cause some lumps, and told that to use milk. So I want to know how much milk I need to use. I am using normal Cadbury Diary Milk chocolate bar approx of 50g.
You do not need any milk to melt chocolate. The goal with melting chocolate is to keep it within a range of heat so that it doesn't burn or separate.
A bain-marie is a technique where you place a heat-proof bowl over a pot of simmering water to heat the contents of the bowl slowly. If you're melting chocolate for dipping bananas, I would recommend using the microwave technique (mentioned in the comments). It's a lot easier. Just do short bursts (10-15 seconds) and stir in between each round.
Aside: If you want to go all out, you can read about tempering chocolate. Tempering chocolate will help the chocolate set with a nice shine. It will also set at room temperature. I avoid all this and throw the bananas in the freezer for a few minutes after dipping them in chocolate. I hope this helps!