how do I underline a word?

This just in, the entire email request:

Hi, how do I underline a word?,

No indication of context, so not sure I could do more than this:

Try selecting the word and pressing cmd and U

How do I use the command line?

Don't use the Terminal/Command Line unless you know what you're doing. You may lose valuable data for ever. I'll try to appear sympathetic, but it will be particularly difficult if you've ignored all my advice about backing up.  

This is the sort of thing you really shouldn't do:

http://www.dailydot.com/lifestyle/apple-secret-bitcoin-mining-feature/

"Am I on WiFi?" (iPhone / iPad)

 Check your phone:

The more white the symbol is, the stronger the signal. 

If you have something else, like 4G, 3G, E, GPRS, where that symbol is, you're not on WiFi, you're using the mobile phone network for internet stuff. If you are abroad, this will likely cost you your firstborn.  You really need to switch roaming off.

"Is this email safe to open?"

No.