In the bottom-right corner of all messages you find three buttons:
Quote,
Multi Quote and
Quick Reply. If you want to quote multiple messages, you
Multi Quote, and if you only want to quote a single message, use
Quote.
Another option is to do it more manually (this is what I normally do). Copy+paste the message you want to quote, and either press

and insert the text between the
[QUOTE][/QUOTE], or type it out yourself. To show which name you're quoting, use
="":
[QUOTE="NAME"][/QUOTE].
In order to color your text, either use the

button in Advanced mode, or type out
[COLOR="COLOR"][/COLOR] yourself. And for striking out text, use either

, also found in Advanced mode, or type out
[S][/S].
Note that
QUOTE,
COLOR and
S all are case-insentitive.