Yes you can use a Samsung 850 EVO 2.5" SSD in your system swapping out your current HD. Follow this IFIXIT guide: MacBook Pro 13" Unibody Late 2011 Hard Drive Replacement
To make this easy get either a USB or FireWire (being faster) external case to hold your new SSD so you can prep it up.
Next you'll want to download from the App store the OS installer.
First you'll want to use Disk Utility to create a GUID partition map and a Mac OS Extended (Journaled) partition on your new SSD. Now run the OS installer and install the OS onto your new SSD. At the end of the OS install process the installer will ask you if you want to migrate your user accounts, apps & data over point to your current HD and all of your stuff will be copied over. The last step is to swap out your drives and thats it!