Tera kya hoga Johny comes from the famous dialog 'Ab tera kya hoga Kaaliya' from the film Sholay. The meaning of the sentence is 'Now what will happen to You Kaaliya,' as in, who can save you now?
The meaning of Tera Kya Hoga Johny, hence, would be What will happen to you Johny. Clearly, Johny is in some big trouble.