There are several ways to make a (part of a) text italic.
If you use the Source Editor you can select the part of what you want to make bold and click on the / on the top of the editor. If you use the Visual Editor you can do this also with the i on the top.
Another way is using this code:
''Text here''. It'll result in this: Text here.
If you want to use tags you can do it on this way:
<span style="font-style: italic;">Text here</span>. This will result in this: Text here.