I would like to say welcome, first thing.
Secondly, I would like to offer this point of advice: Don't let yourself get too carried away with the abundance of stuff to use.
While it's possible to tell a compelling story using clipart, you'll find that most everything here is geared more towards humor than drama. Also keep in mind that the more you put in three panels the harder it is to follow what's going on.
In any case, continue to play around with what's available and get familiarized with how things are put together. Also check out the following links, as they will help you get familiarized with the site and community.
First and foremost, the FAQ:
http://www.stripcreator.com/faq.php
Slightly less important is the In-joke FAQ, so you know what we'll immediately dismiss and why we willl at random use certain characters in certain ways.
http://www.stripcreator.com/forums/showthread.php?postnum=170916
And for a hodgepodge of the above info plus some info on a number of the more active members, the Stripcreator Wiki.