How do I trim this surface
Select and intersect face with selection, but the better question would be why has Follow me created these faces.
I don’t know how to do this I just try the follow, do you have better how to do this I have to create pier of sky train Thank You 
For now let’s forget about doing it better.
To fix what you have right now…
Select the two surfaces forming the protruding parts that you don’t want.
Right-click > Context-menu > Intersect… > with Selection…
The surfaces should now be ‘cut’ with a new set of edges where they were intersecting each other…
Use the Erase tool to delete the edges of the protruding parts.
Now your form should be trimmed acceptably at that junction…
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You got those pieces sticking out because the profiles that follow-me’d up to the corner are of different sizes and you used two paths that meet at the corner, not a single path that went around the corner.
Select the fragments by dragging a box around them. Then right-click and select "Intersect Faces With…->Selection. This will generate edges along the intersection of the two extrusions. Finally, delete the excess that sticks past the intersection. You can use the eraser tool, or select edges and delete. Note: depending on the size of this part of the model, you might run into the effects of SketchUp’s nearby-vertices cleanup, which will leave gaps or holes if it occurs. The fix is to undo the intersection, scale up the model by maybe 100, redo the intersection, and then scale back down by 0.01.
TIG types faster than I do!