One option is to type [command] + [shift] + [p] (or the equivalent) and then type ‘indentation’. The top result should be ‘Indendtation: Reindent Lines’. Press [enter] and it will format the document.
Another option is to install the Emmet plugin (http://emmet.io/), which will provide not only better formatting, but also a myriad of other incredible features. To get the output you’re looking for using Sublime Text 3 with the Emmet plugin requires just the following:
p [tab][enter] Hello world!
When you type p [tab] Emmet expands it to:
<p></p>
Pressing [enter] then further expands it to:
<p>
</p>
With the cursor indented and on the line between the tags.
Meaning that typing text results in:
<p>
Hello, world!
</p>