To make part of the text BOLD, select it and press CTRL+B
To make part of the text UNDERLINE, select it and press CTRL+U
To make part of the text ITALIC, select it and press CTRL+I

Or select the text you want manipulated, select it (like you would in a word processor; double click words, triple click lines, or shift+arrow key) then hover your mouse over the controls above the text-entry box.

See attachments if a 'visual explanation' would help.

If you click the 'raw code' icon (a/A, in top right) then you'll see how the code works.