First off, take a look at the demo site for Selecta and also read through the theme page for Selecta.
On the demo site, you will see three main sections on the front page. The top section is the featured post slider section. To get a post to show in the featured post slider, you have to set it as a sticky post. You have to activate the featured posts feature at appearance > theme options.
The second section is your latest posts section which will highlight your 4 latest posts. You have to activate this feature at appearance > theme options. If you have a featured image set for your posts, the image will be used in the latest posts modules, otherwise it will just pull the beginning of the text into the bubble.
The third section at the bottom is the main content area which will display your posts in the standard blog layout with the latest post at the top with older ones underneath that. Sadly this repeats the 4 shown in the latest posts section above. You will see the sidebar you see at top left on your site in the bottom main content area on the demo site.
If you look at the demo site, you will see that once you get things configured like the demo site, there is going to be very little room for a search box at top right.
What you are seeing in the sidebar of the main content area is a sample set of widgets the theme will display when you have not yet added any widgets. To get the categories widget to disappear, go to appearance > widgets and add the search widget, or whatever widget you wish to the sidebar and the sample set will disappear.