There are many platforms available for starting a blog site either free or choosing a premium plan and WordPress is one of the most popular, reputed and famous site for this purpose. It has free hosting as well as various plans for the prospective people who want to use this platform for content uploading and then making money from it. Before we start as to how to use this platform for this purpose one thing is to be understood very clearly is that there are millions of people having their blogs in different platforms and they are uploading all sort of contents on the internet and they have affiliations with advertisement programs also something like Google Adsense or akin to it as some platforms have their own advertising programs for revenue sharing. So, in this crowd of blogs and websites, only those will be able to earn revenue through the available schemes and programs who have good traffic to their blogs and one thing that is also to be remembered is that as many of the bloggers are uploading the same topic and content the question of relative quality and level also comes in picture. So the most vital part of all this is an extremely good quality of the content and good continuous traffic to it. Why I am saying continuous traffic is for one day all your social media friends will come and look at the content and go but next day again we require the traffic and only one day crowd is not going to help. So, for making an earning, real traffic is required which is technically known as organic traffic.
WordPress has its own ad program known as WordAds and everyone who opens a blog site in it applies for it and once WordPress allows for it then the link for earning money through this ad scheme starts.
Now coming to your specific queries, let us go through them one by one. The ranking in Google is a tricky matter but it mainly depends on the quality of the content, traffic to the site and the frequency with which it is used by the people. Another important thing is whether the site is a general type or is pertaining to a niche area. For example, if you make a site where all sort of health problems are discussed then it will be a mixed lot and people might not be interested to search their problem here. But if you say that people suffering from the liver disease will find a solution here then it becomes a specified area and the interested people will get attracted to it immediately. So choosing the content for one's site is a very important thing and has to be done in an intricate manner.
Next thing is that content is to be added continuously and in an updated and refined version. Many bloggers suffered on this account as they did not update their content time to time suiting to the changing requirement of the users. If you have written an article 2 years back like - 'Top computer programming languages' and meanwhile a new language has emerged on top then this article has to be upgraded otherwise it will go in the backyard.
SEO is a technique of attracting the search engine and that is itself a big subject but in essence, it is the effort made by blogger to attract more traffic to its blog through the organic result of search engine. It is logical that if our blog is reflected on the front page results of a search engine, more people will click on it to see what it is. For this a lot of effort on the content side as well as making it useful and practical for people so that they come here again and again.
It takes time to establish the blog site as one has to make so many corrective actions seeing the traffic and comments of people on improving it. Remember that the visitors to our site are the real reviewers of our site and its worth. We must adhere to their requirements and aspirations from our site. This is of paramount importance.
WordPress does not allow to use Google Adsense as it has its own ad program named WordAds. So one has to use it there.
As regards the affiliate marketing it is related to providing links and banners to other business sites which are selling some products and providing services. If one works in collaboration with them for increasing their sales then they may consider giving a part of that to the affiliate sites. That is a common thing nowadays to attract people to other business sites through affiliate links and banners.
In conclusion, I will say that earning from blogging or site is an effort to make a place in the already crowded market and though there are numerous tools and techniques available for developing one's blog or site but the main thing is the quality of the content, its appeal to the user, and traffic coming to it.
Knowledge is power.