It takes time for Google to crawl your site and "learn" what your content is about. As time passes, and as you update your site, the ads will become increasingly relevant. See this post in the Google AdSense help center: Why Aren't My Ads Relevant for more information about this process.
It might be a good idea to first develop your site and start attracting traffic before placing AdSense on your site. This will give your site more credibility and when you do place the ads, Google will have enough data about your site to make the ads more targeted.
Google AdSense only works if the ads are relevant and targeted to your readership, so Google writes sophisticated programs to ensure that this is the case. However, it takes time and you need to work with their system in order to be successful.
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