First of thanks for using Ridizain for your site and I'm so glad you love the theme :)
I'll do my best to assist you get the site looking like the demo or as close as possible.
Firstly, let start by making sure that all (or most of) the posts have a featured image (post thumbnail) allocated to them. This can be done on the post add/edit page - you'll find the section at the bottom right of the page.
Secondly, make sure that you have a few categories set with a good number of posts attached to them.
Finally, make sure you have at least 3 posts from the above collection tagged with a desired tag for the featured section - for simplicity lets call this tag "featured"
Now to put it all together...
Top section is driven by the Featured content function and this pulls posts marked with the desired tag i.e featured. Visit the customizer and click on the Ridizain Featured Content tab, select the number of posts to show - for now lets keep at 3. Check we have the correct tag in place and that Grid is selected and save. At this point checking the site should have the Featured Grid showing with the 3 posts.
Do remember that the posts selected for the featured section will now not show in the blog feed - unless you select to show them.
Now for the left column (sidebar): Visit the widget section (found under the Appearance tab). Here you'll notice a widget title Ridizain - Recent Posts. Drag and drop this in the Primary Sidebar section, the widget will automatically expand with options to select the desired category, number of posts to show and a few check boxes for extra options. Make your desired selections and click on save - now viewing the site you'll see the a list of posts down the left column.
Do the same for the right sidebar - either use the Recent Post widget above or use the Ridizain Ephemera to showcase your Post Format Types.
Lets start with the above and then come back with other settings if the need be.