Access to and knowing how to use, are two different things. If you were good at sketch up, you would not need to ask. I have seen people make fantastic models with Paint, though it is hard and time consuming. What kind of structures do you wish to make?
For more I suggest you use the search function in the forum, check out the Architectural thread, and do the same with Google. The following results were garnered with the wording "FREE architectural models" on Google and I got all the results below, on the first page, not even the first page because there are too many to list! Be a little more specific with what you are looking for or trying to do and we can help you out better. All the models I have pointed you to are FREE. This forum's focus is on the FREE offerings of models sites that give their work away. We do not allow commercial listing. You would need many lifetimes to build all the quality free models that are available.