To create a component or group, select all the faces and nodes of the bits you want in it. (It's usually easier to create models in separate files so you can just CTRL+A to select all. I found this out the hard way. Selecting only what you want to select in a larger model is time-consuming and I frequently missed bits that were integral to the model.) Right click, and you should get the option to "create component" or "create group". For my lower class housing, I'd have a roof component and a house component, then fit them together and made them into a larger component. You can then group clusters of models so they're easier to move around together.
It does make editing more time-consuming because you have to right click on a component/group, then edit, then start the actual edits/drawing - but it also saves on mistakes, because you're not integrating the component model into the larger model. (I've got a terrain component in my map with building components/groups on top. I originally tried drawing them without the components and any edits were costly. Deleting one thing could ruin the entire map by deleting faces or lines I didn't want gone.)
The best part is that you can save models as components and then import them into other files.
I like the lines on your dome. It helps show the shape, so I'd leave them in (especially if your dome is has a metal framework that suits the same lines). I don't think the shape would be nearly so clear if you removed them.
Trees... you can find models for trees in the Sketchup model database (I forget offhand how to get to it, but it's one of the default toolbar icons, I think. Import model or something like that). They're flat so you can only see them from one angle (some are sideways, some are top-down), but might still give your model a bit of colour.
Fields, on the other hand... As far as I know, you'd need to draw out the sections that are fields and then change the colour or texture. You could use a vegetation texture, maybe? Or just a deeper green since your model's just simple colours. (I like it that way, by the way. I think textures would ruin the style you've got.)
