How to talk about raises?

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I don't like my job a little but I need money, so I had to keep on. Recently a colleague resigned and he will go at the end of August, I took over his job since last month. His salary is higher than me, so I want to get a raise as my work is much more than before. I am not good at this, so I want your opinions. How to start this conversation and how to talk about it? My dear friends. Thanks a lot firstly.