There are 3 basic rules for sketching outdoors:
1. First, determine your horizon line, making sure that all your receding lines converge properly.
Sketch the general contours of the big shapes. Keep them fairly angular and avoid drawing details too soon.
2. Second, the same advice applies to shading.
Indicate the large masses first, in simple light and shade.
Avoid using halftones at this stage. Watch your source of light and keep it consistent.
3. Third, put in the half-tones, constantly checking the relationship of the light and dark values.
Then render the details and necessary accents in the final stage to complete the picture
As you can see, these rules are very simple and easy to follow.
Now go out there, and do some sketching!