Begin by making a value scale.
1) Paint the whole strip #9 (the lightest grey), except for a patch left white at one end.
2) Let the strip dry, then paint the whole thing #8, except for a patch of #9 and white.
3) Let it dry, then paint the whole thing #7, except for the patches of #8, #9 and white.
4) Continue getting darker by increments, always leaving a patch of the previous layer.
Don't leave white between your patches.
Everyone worked on a deliberately over-simplified monochrome study of a color photo yesterday. It might be a good idea to do that again, using the same image or a new one. I'd like you to work much faster eventually, so you will be encouraged to do plenty of preliminary studies. Practice, practice.
When you have a feel for the dark/light relationships of the major shapes, try making a color version that stays true to the values. Keep it too simple. See if you can let go of texture and detail, giving emphasis to shapes. That will make it easier to focus on value as a feature of color.