How can I find which app slows down my phone? I have a Samsung Galaxy S3.

My phone is becoming slow. I don't know how to find the app that slows it down. Can someone help me with this? My guess that the app is FACEBOOK... worst app ever.

After restart, the phone works fine for some time. Then, it slows down again and becomes lagged.
You could try to use some live monitoring app, e.g. OS Monitor. These usually offer you a view on apps currently consuming most of the ressources, e.g. hogging your CPU, eating your RAM, or utilizing your network:

 
OS Monitor showing running processes, and showing network connections (click images for larger variants)

Running processes can be sorted by several criteria, such as name, CPU usage, and more. The device being "laggy" most times means some app consumes all CPU power, or I/O, or RAM – so this should help you figuring out.

An alternative to mention is SystemPanel, which even offers you a manual (for your situation, the system monitor page would apply). In its payed version, it even offers you background monitoring, so you could first let it collect data for a while, and later check for the "largest consumers".

 
SystemPanel: running processes, overall ressources (click images for larger variants)