Things to try… most simple is just adding an edge split modifier, if the thing has been smooth shaded those lines could just be the shading trying to cope between the 90 degree angles connecting the front to the sides.
If that doesn’t work you could try ctrl + T also to triangulate the large front face and see if that fixes it. If not, you’ll need to undo that and start cutting the large n-gon up in to smaller pieces by selecting two vertices and pressing J for example to add an edge that split the face, so that the individual areas of face dont end up being concave or having to go round too many corners.
Is there a certain reason why you converted the bezier curves to mesh? You could just have used Blender’s curve properties to get a three dimensional impression of your curve:
Hard to see what OS you use, but it looks like something from the Windows family: Hitting the “Print Screen” button will make a screenshot image of your desktop and store it in the clipboard. So, no need to actually photograph your monitor…