do you want to upload the app to Google Play?
if not, then use the texting component together with a clock component to avoid sending the sms all at exactly the same time
EDIT: see also The ‘Do something’ example
A very good way to learn App Inventor is to read the free Inventor’s Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps … the links are at the bottom of the Web page. The book ‘teaches’ users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also do the tutorials Tutorials for MIT App Inventor to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun