There are different ways to emphasize words in your text. But before you think about how to do it, consider when to do it. Most fiction writers use emphasis in dialogue. It portrays the way something is said as well as exactly what is said. Sometimes, you'll want to use emphasis in your character's thoughts. You'll rarely use it in narrative.
Before the advent of word processing, putting emphasis in text was much different than it is today. Writers today have various options.
Some writers still use capital letters to emphasize text. Most editors don't really like this. There are some cases where capital letters are used in text (besides proper nouns or the beginning of sentences). These aren't so much for emphasis as distinction: academic degrees (M.D.; Ph.D.; M.A.); initials that take the place of a proper noun (J. P. Magnus); states or countries referred to by initial (NC or USA).
Showing emphasis this way was popular when people used typewriters. But today, we have so much more.
Basically, italics is a style of type. Its slanting letters create emphasis by appearing different from the text around it. Most word processors do this fairly easily. Just look for the big I in your toolbar.
The beauty of italics is very simple. It appears to show the tone of speech, then disappears. Readers have come to know what this means. If it's in dialogue, it shows readers what words or thoughts carry particular emphasis. It's also come to represent thoughts shared with the reader directly from the point of view of the character. In science fiction or paranormal fiction, italics are frequently used to create the impression of telepathy (mental communication).
Another type of emphasis is using bold type and underlining. Fiction writers rarely use these in text, and underlining is particularly discouraged. If you want to be sure, check the submission guidelines of the publishers you're planning to submit your work to.