Linking forms together

Question

Could anyone help me and tell me how can i link two forms together. I've tried putting .Show() on the first form and then getting the new form in a new tab, but that's not what i need. What i actually mean if i press a button on the first form i need the second to start inside the first one. Actually to make 2 different forms into one.
For example if i press play on a simple game, the game starts instantly in the same dialog not a new one.

Thanks.