For activate and select github blog for website, you need to settup your site, visit Settings - Themes - and click activate on Github Blog.
And you can configure title and description for your site to , visit on settings - general - insert title and description website in here and settup all you needed in here.
For write content, you need to create categories for grouping your article, then you can create and write your content for blog post or page, you can change article content with your own, just click on options button in , then click on advance - you can select article content type in here.
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