When a web browser interprets an HTML file it ignores all the normal line breaks and tab characters in the document treating them as just spaces. It then displays the text ignoring duplicated spaces wrapping the text wherever necessary to fit the current browser windows width.

The HTML tag to force text to start on the next line is <br>

Which forces a new <br> line like this

To start a new paragraph the HTML tag is <p>

Which looks like <p> this

There are many ways, in HTML, to emphasise sections of text or words.
To make a section of text bold the tag is <b>

<b> Bold text </b>

To make a section of text italic the tag is <i>

<i> Italic text </i>

These effects can be combined

<i><b> Bold Italic text </b></i>

It is good practice to always close all tags (even at the end of the document) and to close the most recently applied tag first i.e. in the example above the most recently applied tag is the bold tag so that is closed first

Another useful tag for splitting up sections of text is the horizontal ruler <hr>

Some text <hr> More text

Some text

Text is normally displayed left aligned, it can also be centred <center>

Note the American spelling