how can you make plastic sink and steel float?

Answer 1

Melt the plastic so it is denser than water,
Form the steel in tubes like drums / pontoons on a boat ,
The air inside floats the steel .

Answer 2

Place the plastic in a fluid that is less dense that the plastic. Some plastics are dense enough to sink in water (ie. your plastic plates and cups, melamine for example). For lighter plastics try a lighter fluid such as oil (to find a lighter fluid try placing it in water to see which one floats ie: oil on water).

For steel, you can either place it on a more dense liquid such as mercury or you could enclose the steel so that the bulk density of the steel enclosure is lighter that the fluid such as a pontoon, as was answered previously.

Answer 3

Make a steel ship, it will float

Answer 4

Sure.
Form it where it displaces the water equal to its weight.
That is reason why steel ships float.