Paragraphs get created as you type and hit ENTER, which will automatically insert a full line break and create a new line for the next paragraph.
If you don’t want to create a new paragraph, but simply want to do a single line break, hold SHIFT on your keyboard and hit ENTER to do what’s called a “soft return.” Otherwise, a “hard return,” or just hitting ENTER, will create an additional line break in between the two sections of text as mentioned above. If you hit SHIFT+ENTER after a link, it will carry the link to the next line. If you want to go to the next line without the link you will need to hit return type in your text and then go to the beginning of the sentence, delete the space and hit Shift Return. This will give you a line break with no link.
To bold, italicize or underline text, use the B , I, or U icons on the toolbar after selecting the appropriate text.