a=5 cm c=6 cm angle B=11.8 degrees what are the step I need to know according to:0.5*a*c sinB

You are finding the area of a triangle, using the formula:  Area  =  0.5·a·c·sin(B)

You know the values of all the variables that you need:    Area  =  0.5·5·6·sin(11.8)

Enter this into your calculator, and you will have the answer.