Perhaps you mean Paris Gare de Lyon station? I've never heard of a train to Milan from Paris-Bercy station (Bercy is metro station).
In that case, to get there you can i) follow MorganB's suggestion of the AF bus to Gare de Lyon, or ii) take RER B from CDG to Chatelet/Les Halles, then transfer to RER A eastbound for one stop to Gare de Lyon. Prices, as he helpfully indicated, would be 12 and 7.85 euros, respectively.