SDL_BlitSurface takes 4 arguments, first one is the source surface or the surface you want to place ontop of your screen ( the image )
The important part is that the 3th argument is the destination argument or the surface you are drawing too ( the screen)
as a side node If the secound argument is NULL, the entire surface is copied.,if the 4th argument is NULL, then the destination position (upper left corner) is (0, 0).
In 1 word your function should look like SDL_BlitSurface( hello, NULL, screen, NULL );