A simple search in Drupal 8

Add a new view:

/admin/structure/views/add

If you are outputting content instead of fields, check the Commit to use fields box in the format settings.

Add the fields to be searched:

In the Filter Criteria we add the Combined Fields Filter field and make it expanded.

In the "Operator" select box, select "Contains". 

In the "Select fields to filter" field, while holding down the CTRL key, select the fields to be searched.

Save a view.

Go to the search page:

The search form is linked to the page. To make it a separate block:

In view settings → Advanced → Expanded form → Expanded form in block → Set radiobutton to "Yes".

In the block diagram:

/admin/structure/block

Add a search box for our view:

I put it in the sidebar:

If you need to make a very simple search, this approach is for you.

Otherwise, I would advise to look in the direction of SEARCH API.