How did Tom charge Gatsby with making his money? What motivates him to do this?

Tom says that Gatsby was a bootlegger -- someone who sold illegal alcohol during Prohibition.  He also accuses Gatsby of having been a racketeer and a swindler.  This happens in Chapter 7.

The reason that he says this is that he does not want his wife, Daisy, to go off with Gatsby.  He is trying to show Daisy that Gatsby is really just a criminal.  If this is true, then Daisy will, presumably, not want to go off with him and she will stay with Tom instead.

So Tom is saying bad things about Gatsby to try to discredit him in Daisy's eyes.