When should I start my preparation for the 2018 university applications?

Now! Yes, you might have already been late to start your preparation if you have not done anything yet. A big difference between the university applications of the US universities vs. the universities in the most part of the world is simple: the former has a much earlier deadline than the latter. Based on the type of universities, you will face the application deadlines as early as November 15th, 2017. If you also consider applying as an Early Decision or an Early Action candidate then your application deadlines may hit even an earlier date, such as October 15th,2017.

So, please do not take things easy and slow just because your class start date at the US university shows some time at or around the end of August 2018. If you do not meet all the application requirements and submit them all by the application deadlines, then you may not even have a chance to think about applying for the universities and programs for which you could and would be qualified otherwise.