You can't simply apply normal map because each normal on that texture should be transformed into so called tangent space and only after that applied to object's normal. Tangent space is a 3D basis built for each object triangle on three vectors - normal, tangent and binormal. So you need to calculate these vectors and send them to videocard with other model data.
When in fragment shader you pick normals from normal map, transform them into tangent space and combine to triangle's normal. After that you can use the resulting normal for lighting calculations.