If my spherical eyesight no. is -1.5 and cylinderical no. is 0.5, then what power lenses should i get?

Simple formula: Spherical equivalent=sphere+1/2(cylinder). The sphere is the first number written in a glasses prescription. The second number is the cylinder. The third number is the axis. Keep track of your signs both positive and negative when you are adding or subtracting above. The axis plays no role in formula above. You need -1.25. Fun fact: circle of least confusion equals spherical equivalent.Google it.

-1.5 or -1.25: Assumption the cylinder power is +0.5 and you getting contact lenses, either -1.25 or - 1.50 should be fine for you. If you get glasses, it should have both the spherical power and the cylinderical power in it with the correct axis (orientation of the cylinder).