When you edit your content in WordPress, you are primarily working with the WordPress visual editor. The visual editor looks much what you would find in Microsoft Word or another word processing program. There’s a large area where you can type and edit your content, and a row of icons above that for formatting.
Here’s what a clean WordPress visual editor looks like (this one, in fact):
If your WordPress visual editor looks more or less like this, then all is good in the world.
However, if it looks like this…
or this…
or this…
…then your WordPress theme — or your website developer — is getting in the way of your content. You may want to consider switching to a different theme — or a different website developer!
See Using the Visual Editor on WordPress.org for detailed instructions on how to edit your content.