@jarballa16 The concept is the same as a dedicated security system. You use door/windows sensors to protect perimeter areas. Motion sensors are used for indoor “area” protection, to detect movement in the portion of the room covered by the motion sensor. If you have pets, make sure your motion sensor is listed as “pet-immune” meaning it won’t be activated by small animals, but it will be activated by a person. Some have jumper switches used to control the sensitivity, and thus the size of the pet detected. Also, be aware that with motion sensors, some have a reset period of several minutes, meaning once activated, they can’t be activated again until after the reset period. Once you have all your motion sensors in place, just set up the app in ST to alert you when they are tripped, either via siren activation, text message, push alerts from the app, call your cell phone via link to IFTTT, etc. Be sure and test, test, test!
Personally, my hardware consists of (all Zwave) Ecolink door/window and motion sensors, and Aeon Labs recessed door sensors, and an Aeon Labs Gen 5 siren. I’ve not had any problems with them yet, but they have not been in use very long.
There are typically 3 security modes: Armed Away is where every sensor is armed. Armed Stay is where only perimeter sensors are armed, not motion sensors, to allow for movement inside the house (get up for a glass of water) when everyone is home for the night. And finally, Disarmed. You can customize each of these for your personal use, or add more modes if you like. For example, you may want to activate a motion sensor for the downstairs in the Armed Stay mode if your bedrooms are on the second floor of a two-story house. All kinds of possibilities.
Smart Alarm is a great app created by @geko here in this community. It is a very popular security app. In V2 of the SmartThings app, ST launched Smart Home Monitor, baked into the app. It is new and, in my opinion, not as comprehensive as Smart Alarm, but perhaps someday it may be. You could also “roll your own” by using any combination of event alerts in conjunction with sirens/device notifications, but the two mentioned are a simpler/cleaner approach. But like I said earlier, neither of these (at the moment) will run entirely locally, meaning without internet service to connect to the ST Cloud. So all an intruder has to do is cut your power line and cable/DSL line and you’re SOL; not what I would call “secure.”
Although the power issue is somewhat mitigated by the battery backup in V2 hub, you would be better off putting the hub on a high-quality UPS backup system. You can get around the cable cut problem by installing a failover router with 3G backup. Essentially, it’s a multi-WAN (wide area network, as opposed to LAN local area network) router that will immediately switch to its backup WAN when the primary fails. So you might have cable as your primary internet, but also a cellular 3G/4G modem as backup. On the better devices, the failover is immediate, so you wouldn’t even know when it happened if you were on a VOIP call. A good router like this will start at around $300. This Peplink router gets very good reviews (and probably my choice) but Cradelpoint, TP-Link and others also offer them.
Even with 100% internet uptime, you are still dependent on the ST Cloud servers which have had a lot of issues lately, though this may improve if/as more operations shift to local operation on the V2 Hub. If your alert/siren activation is delayed by even a minute or two, you might wake up staring down the barrel of a bad guy’s gun, or even worse, not wake up at all.
So bottom line, even ST V2 Hub is NOT a security system I would bet my life on, at least not until EVERYTHING can run locally. And even then I have a backup 
As a newcomer to home automation and SmartThings, I can tell you there is a TON of good information you can learn by perusing this Community board, and by using the search tool (magnifying glass upper right) to target specific keywords you’re interested in. And if, after you’ve searched, still can’t find an answer, just ask away. Most everyone is helpful, and some of the threads can be quite amusing.
Also, know that new custom apps are always being developed by many of the members here. They frequently involve some effort (cut & paste code) but not that much, and the good developers give explicit instructions on how us mere mortals (non-coders) can implement their work. Without their work, ST would be nowhere near what it is today.
Hope that helps!