If you want readers to see a particular post ,message ,announcement when they visit your website ,you need to make this post sticky post .To make a sticky post ,you can do several ways ,you can edit the template for showing a specific post ,or easier ,changing post date.
Change the Post Date Method

Posts in blogspot is ordered as time order . If you change your post date to a future date like the year 2015 ,the post will stand in first position permanently .
Open your post editor and change the date to future . Done

Open your template ,and insert this code where you want searchbox appear :

<form id='searchThis' action='search' method='get'><input id='searchBox' name='q' type='text'> <input id='searchButton' value='Go' type='submit'></form>

Another way easier . You can insert a Html/javascript widget in your site and paste the code above in the content of widget .
You can customize how searchbox look by adding some CSS 
Done !