That’s a reasonable question 
!
SmartThings is getting closer and closer to having a rapid way for Community Developed SmartApps to be certified and shared right in the mobile user interface under the Dashboard “+” pages.
Right now, for example, under “+” / SmartSetup / More / Safety & Security; you can install a SmartApp called “Smart Security” … “Alerts you when there are intruders but doesn’t sound a false alarm when you get up for water in the night …”.
But while that is “close” to what you need, I’m guessing it isn’t exactly the best SmartApp.
So … instead you can read up on a, so far uncertified, but shared, SmartApp called “Smart Alarm” and install it via your API - IDE login (gosh … we need a FAQ on that procedure … it’s written up a dozen times – really pretty easy).
Take at look at this Topic and either experiment and/or come back here for more details, ok?
You may find installation instructions in that Topic or via Forum Search, or can also Private Message the developer… This is an ad hoc process, but the results are worthwhile.
…CP / Terry.